"Google leveraging a middleman role to maximize revenue extraction from both sides of a transaction it facilitates" is exactly how I would describe the google ads business.
I do think the dynamic there is still a bit different from where the term usually applies, since advertising involves three parties in addition to the middleman, not just two -- the above comment was looking at things from the perspective of end users, in terms of where they prefer to watch videos, and that actually may represent a constraint on how much "enshittification" can happen that's particular to this industry.