A stripe customer is free to tell Stripe "Your fees are too high, and I'd prefer switching processors than raising prices".
I could be wrong. Is there an example of an illegal cartel, trying to persuade customers from whom it makes transaction fees, to raise prices? Surely this isn't a new thing under the sun.
The issue is that coordinating pricing is illegal in any form. In any industry, each company could make more money if they knew their competitors would raise prices. Even calling a competitor to tell them that you are raising prices is illegal if they raise prices.
If there's even a single incident of two companies being advised to raise prices on products which compete with each other, it's about as close as it comes to an open and shut price fixing case. And this post makes it easier to prove because it suggests that companies know he's giving the same advice to their competitors. Whether that's said explicitly or not won't matter.
It's the raising of prices in the absence of market forces that gives room for the payment processor to increase fees. Theoretically all payment processors benefit from advocating that their clients raise prices. If some companies raise their prices, no issue, but the coordinated effort to do it is where you "smell a rat".
It's then up to the litigating party to try to find collusion between companies to do this. This is like how the anti-competitive hiring practices of Google/Apple/Facebook/etc were surfaced. They all had a set of practices that stood out as anomalous and during the subsequent investigation it was found that they colluded with each other to set the market (rate for hiring talent).
It's probably not the case here...and also patio11 has little risk saying so while living in Japan where industries are highly vertically-integrated/monopolistic.