Every time this issue comes up, a bunch of people crawl out of the woodwork trying to prove how "wise" they are by mouthing this idea about chargebacks. And the processors are happy to keep their heads down and not dispute it, or even encourage it, since they really want the whole issue to just go away.
Chargebacks are not the issue here, and if you haven't paid any attention at all to what's actually going on, you're best advised not to make yourself look like a fool by talking about what you guess might "most likely" be happening.
That's definitely what I would claim if I wanted to take down content I didn't want. Who is going to prove them wrong?
Contrary to popular belief, corporations are composed of humans and do not reliably or mechanically follow only financial incentives. Nor are they always perfect in understanding their actual financial interests. Boycott threats are probably empty, but some people may not want to take the chance, or may have other motivations that cause them to overestimate that risk.
I would wager it is closer to "economically load bearing" than it is to peanuts
Ideally, you could take your business elsewhere. The problem is providers that handle these industries will expect a premium across all transactions and it would come off badly when customers see that large percentage added to each purchase.
So you have the decision of: your customers pay a premium or you don't carry adult material. If all your business is adult, it's an easy choice. You pay the premium.
This activist organization is pressing them until they are forced to make that decision based on a small amount of their hosted content. This is what I see as likely. Admittedly though, there are not enough details given to say for sure.
Besides it not being a valid reason at all, they are not even trying to claim chargeback costs.
You think chargebacks are disproportionately higher on NSFW games revolving around non-consensual themes versus other fetishes? Give me a break.
Payment processors have ways of passing some of the chargeback risks onto the stores, and it's not like Steam itself is chargeback central. If you just want free games, pirating them is extremely easy, and trying to abuse chargebacks gets you banned.