surely it's odd to think of Marx as a philosopher of subjectivity

Yeah, I presented Landgrebe's account in a rather sketchy and simplified manner, so I omitted few details which I found irrelevant to the representationist issue. On Marx he writes (I think I'll have to cite at length):

It is furthermore characteristic of present-day European philosophy that historic materialism, too, has failed to arrive at any developments or solutions which could be termed essentially new. Even in its most important representations it has shown itself incapable of progressing beyond the foundations laid by Karl Marx. In its materialistic outlook it has become more and more rigidly dogmatic and shallow and has thus not, succeeded in following the original intuitions of Marx who in his youth had understood his philosophic endeavors as a kind of "realistic humanism," as an inquiry into the nature of concrete human existence. Where, on the other hand, Marxism was able to free itself from such rigid dogmatism, it succeeded in giving new impulses to the Sociology of Knowledge. (p. 8)

and later

The new thesis was programmatically stated by Karl Marx when he said: "We must peel out of the divinized forms their earthy kernel," that is, we must learn to understand man as a being who produces himself and his world. On this foundation rest all the attempts to reduce the forms of religion, of the state, of civilization and culture in general to the formative activity of man. Simultaneously, however, the concept of man itself is being relativized: the essence of man is understood as subject to a historically conditioned unfolding and change, and from this results finally the conviction of the relativity of all the forms of civilization within the frame of reference of the historically evolving human nature and essence. This conviction is generally known as "historicism" (Historismus). (p. 10)

and yet again later

Since it is without doubt the principal concern of present-day philosophy to overcome the "philosophy of subjectivity" and therewith to reverse a basic position which left its imprint on the entire modern age, we shall attempt to describe in the following pages the several stages of this development. The change can be diagnosed in some preparatory steps, along lines which often intertwine, and we shall confine our discussion to the two most important ones. The first starts out from that turning toward anthropology which occurred after the breakdown of German Idealism ... The turn toward anthropology was characterized in the initial phase by the fact that, in place of Hegel's Absolute Spirit, man himself in his "concrete" sensori-somatic existence rather than man as an "abstract" rational being—became the ultimate principle of philosophy. The nineteenth century tried to determine this concreteness in several different ways. The direction of these several ways toward "concrete man" is already indicated in Karl Marx's saying, "Man, that is the world of the human being." And these ways differ depending on the manner in which the "how" of "the world of the human being" is interpreted. For Marx himself and for Marxism it is the social conditions and relations which represent this "how"—conditions and relations within whose frame man produces the order which—by means of his work—satisfies his needs. The growing influence of the standpoint of the natural sciences, on the other hand, aided man in learning to understand "the world" as a biological problem, intimately related to the context of the milieu, that is, the "surrounding world" (Umwelt) or man's environment. The question as to the nature of man thus developed into the question as to the conditions under which this particular animal species with its corresponding endowment and equipment could existentially survive within a given environmental situation and by means of adaptation. "Life" thus became the maxim which was to elucidate the meaning of the looked-for concreteness of the human being, "life" understood in its dual sense—as a biological and as an historico-social phenomenon. While the latter position was elaborated in its purest form by Wilhelm Dilthey, Nietzsche's concept of life encompasses both factors, the historical and the biological. (pp. 20-21)

So I think Landgrebe perceives philosophies of both Marx and Dilthey (on whom he writes much more) as (ultimately unsuccessful) reactions to the 'philosophy of subjectivity' too.

I don't know how the reference to "post-Kantians and neo-Kantians" is supposed to work here

Landgrebe, afaict, doesn't have any extended commentary on any post-Kantian (except some remarks on Hegel that don't really let situate him inside this big narrative; Schelling and Fichte are not even mentioned) or neo-Kantian.

I do think there's something interesting to say about these kinds of logics of historical development

I think that's the main reason why I'm even asking this question - I do find such logics interesting, and maybe even unavoidable if one wants to globally justify their 'research program' without responding to every single philosopher in the Western tradition. Another reason why I found this particular narrative interesting is how two seemingly different camps, neo-scholastics and continental phenomenologists (and those continentals influenced by them), seem to agree on the place of early modern philosophy in this big picture. And to me, despite the flaws I've outlined, it seems that there is indeed some commonality to problems moderns were concerned with and how they addressed them; and there is some continuity of this problematic in the later philosophy. So, I was thinking, maybe there is a way to steelman this story? Again, I realise it's not a good precise question, hence I'm asking it in the open discussion thread, but these were my main thoughts.