Valve confirms credit card companies pressured it to delist certain adult games

if I was doing a couple of billion a year in transactions then the payment processor would be told where to shove it

A couple of billion is an insignificant fraction of the $10000bn MasterCard processes every year.

which is relevant how exactly?

merchants don't deal with mastercard, they deal with an acquiring bank

of which there are hundreds

no doubt one of which will be happy to take the business

Mastercard appears to be involved in the pressuring. You can't avoid them.

certainly not explicitly mentioned in the article

and I very doubt it's the case, the card networks simply don't care, given you can buy adult entertainment from millions of websites

the acquirer will care if it pushes up their chargeback rate, but this is normally solved by the merchant by paying a couple of bps more

it's a negotiating tactic, nothing more