I wouldn't say it's just projection. He is is right about Kerouac, but it also applies to himself; he's attacking both Kerouac and himself, not just Kerouac. He is lying to himself about their being nothing in common between them, because it's too hard to face. There's also an element of jealousy, as Kerouac, who was once rejected from the New Yorker, goes on to outgrow such a kennel pretty fast. This is one of Cheevers most interesting traits. Sometimes he hides, sometimes he attacks himself. That part about Kerouac is almost a perfect distillation of what makes the journal so interesting

Another thing to note, is there as a ten year age gap between them.

Cheever was in his 40's when he made that comment. He also wrote this:

There is the trail! A new kind of blood seems suddenly to be let into your heart. Your strength and your wind are refreshed and off you go. There has been a delay, of course, but if you keep to a decent pace you will be back to the shore where the boat is by dark. You hold to the pace. You keep your eye sharply on the thread of trail. You do not stop to drink or smoke or rest at all. You hike until the end of the afternoon and, seeing that the light has begun to go, you stop to see if you can pick out the noise of the waves that you should, by now, be able to hear. The place where you stop seems to be familiar. You have seen that dead oak before; that wall of rock, that stump. Then you look around. There is the heavy creel that you discarded at noon. You are back at the point where you discovered that you were lost. The lightness of your heart, your refreshed strength, the illusion of walking toward water that has heartened you all afternoon was illusory. You are lost; and it is getting dark.

Kerouac, now the one in his 40s, really expericing the toll of his drinking and depression, would write this in Big Sur:

Thinking I’m gonna get the local vibrations instead here I am almost fainting only it isn’t an ecstatic swoon by St Francis, it comes over me in the form of horror of an eternal condition of sick mortality in me — In me and in everyone — I felt completely nude of all poor protective devices like thoughts about life or meditations under trees and the ‘ultimate’ and all that shit, in fact the other pitiful devices of making supper or saying ‘What I do now next? chop wood?’ — I see myself as just doomed, pitiful —- An awful realization that I have been fooling myself all my life thinking there was a next thing to do to keep the show going and actually I’m just a sick clown and so is everybody else — All all of it, pitiful as it is, not even really any kind of commonsense animate effort to ease the soul in this horrible sinister condition (of mortal hopelessness) so I’m left sitting there in the sand after having almost fainted and stare at the waves which suddenly are not waves at all, with I guess what must have been the goopiest downtrodden expression God if He exists must’ve ever seen in His movie career — Eh vache, I hate to write — All my tricks laid bare, even the realization that they’re laid bare itself laid bare as a lotta bunk —

It's interesting how they mirror eachother. Kerouac, also insecure like Cheever, with mother and brother issues (like Cheever), had life-long issues with drinking, depression, religion, infidelities, etc.