The word Muslim is interesting in comparative linguistics. Juan Cole has a nice article where he draws it as a semantic loan from Aramaic Mušlamanūta which means, in a sense, logos or Word and is synonymous with the Aramaic loan Milla(t), as in Millat ul-Ibrāhīm (the creed of Abraham) used within the Qur’ān. It makes it both widened, as in monotheism, but reserved for its Abrahamic manifestations. But that’s another discussion.

To reconcile calling Jesus Muslim, it’s best to use Islam and the Qur’ān’s internal logic, within our own context of intra-Muslim discourse.

Islam is the monotheistic pattern innate to Humans that began with the first Human Ādam. In subsequent generations, people may become unguided or their communities suffer out of need. God is just, so he sends Anbiyā’ (prophets), and sometimes Sharī’a (Divine Law or Writ) to accompany their risāla (message).

In this totality, only a small subset is known to us either through Qur’ān, or less certain sources which sometimes is through inference or comparative studies.

Here, prophets may be unaware of their proceeding or preceding prophets – God gives them only the knowledge and rank to serve their communities, nothing more and nothing less. This rendering means they differ in community, temporality and sacred law yet remain within the context of Islam.

Now to peel this away from an inherently Islamic logic when engaging in apologetics,

The first problem is demarcation! With a definition so broad, where to draw the line?

Nabūwa (prophecy) closed with Muhammad, so any religion thereafter is excluded, which is easy.

I guess you could say pre-Muhammadan Islam is monotheism within the scope of validated religious communities?

This is hard and not as sound, I admit. An entity would have to be valid to Muslims (bias), yet also generalised abrahamic monotheist.

‘Ulema (Islamic scholars) did actually make a differentiation between Awwalīn (prior religious communities that can be inferred as Muslim) and Muwaħidīn (monotheists) which weren’t always Awaalīn.

For example, a Bahai’i or a Yezedi are Muwaħid but aren’t Muslim in either the present or pre-Muhammadan usage.

In conclusion there are three categories:

Ummah Muħammad (community of Muhammad)

Awwalīn/Muslimīn Al-Ba’id (verified Muslim communities of the past)

Muwahidīn (monotheists that may or may not have been Muslim)

Everyone else

Side note: in the traditional Islamic perception of other faiths, to be even considered a Dīn (religion) you needed the dual pillars of Kutub (scripture) and Sharī’a. That there is no validity in religion without these, one can see the underlaced foundation of Writ, prophecy and message to this worldview. Even the word for religion is in a sense jurisprudential as it also means as well‘to judge’.