When you consider how much traffic goes towards the larger sites, it's actually believable. Even before the great migration Reddit was infested with reposter bots whose sole purpose was to farm karma in order to later sell the accounts. Those bots have gotten more sophisticated now, replicating not only original posts but entire comment threads. That's not new content, but it's content nevertheless, especially in the context of the dead Internet theory. Yes, it's engagement farming, but that engagement is getting more sophisticated, both to trick the user (to drive engagement) as well as to trick the server (to prevent getting blocked).
This is a very insidious problem, because it means that such bots can and will be abused by threat actors (both internal and external) to drive popular sentiment in certain directions. We know how susceptible a generation that only watched cable news became, imagine what such campaigns can do to internet generations - if you can generate content that supports your rhetoric faster than humans but without appearing fake, then you can drown out dissident speech. Brigading is bad already, and it will get worse.