It is not hugely surprising to find, as Andrew Piper and Chad Wellmon have done (the full pre-print is linked via IHE), that elite journals tend to be occupied by authors from elite universities. Putting aside the hypothesis of the relative inferiority of scholars like yours truly--I hail from an elite university, but don't teach at one, ergo...--the material conditions of production, as some scholars like to say, do partly account for the results. As some of the IHE commenters also pointed out, if you have research funding, a well-stocked research library, and a 2-2 teaching load (or, as at the University of Chicago, a 2-1-1 teaching load, or even a 2-0-2 teaching load), accompanied by tenure requirements that concentrate the mind wonderfully, then of course you will write more than someone who has none of these things. (One might add that SLACs are infamous for having murderous service requirements, which also cut into research and writing time.) Moreover, faculty at R1s may experience overt and covert pressure to write about certain topics and publish in certain venues, whereas someone outside the vaunted halls of ivy has the liberty to write whatever they please and publish wherever is appropriate. For example. PMLA is really fond of heavily-theorized, single-author and/or single-text readings, which I...don't do. Ergo, it has never occurred to me to bother to send them anything--instead, I aim for literary-historical venues hospitable to work on religious authors of whom nobody has ever heard. Another factor, too, may be in play, and was when I was working at Modern Philology in the late 90s: the shift to writing commissioned articles for edited collections. (We were genuinely terrified at one point that we wouldn't have enough articles to publish an issue.) A good chunk of my own recent work has been commissioned, which means that, by default, I'm not working on projects intended to make the rounds of any kind of journal. In other words, I have established a reputation (people ask me to write for them) but work in a niche field (looks puzzling to the editors of CI). It is highly unlikely, to be honest, that my own scholarly interests could have developed as they have had I started at an R1.
What about other prejudices? There are certainly horror stories out there, some of which go back decades; this evening, my father told me an anecdote relayed to him by his own dissertation supervisor, who had been rejected by Very Elite Classics Journal in the 1940s because, as the editor explained, he wasn't important enough for them to publish. (The article appeared elsewhere and wound up being highly influential. Oops.) Certainly, it remains the case that the "blind" part of the peer-review process begins with the readers, not the journal editors, and a snooty editor might well give the heave-ho to an article before it ever made it out of house. It's rather difficult to quantify such stories, though.