Plenty of proof and devs talking about it in the article and on Twitter. Rami Ismail is well known and has been pretty clear about how much overhead these types of a abusive companies cost them.
It's hard enough to be an indie without having to deal with this.
A problem that costs 30% of revenue is something that I would consider a hard problem to deal with.
They want to sell their game through their own storefront to avoid the platform fees, but they don't want to deal with chargebacks or fraud.
They want to give away hundreds or thousands of free keys, but don't want the keys to be resold and don't want to exercise due diligence when giving the keys away.
They want to recoup their losses when fraud does occur, but don't want to expend the effort of sending G2A evidence of the chargeback to get the 10x refund.
This is called being a business--each dev needs to decide whether avoiding Steam's cut is worth the headache of being your own seller. Trying to fight the entire concept of reselling itself is a much more time-consuming and futile effort than just choosing one option or another.
The fact that indies are saying they'd prefer the game to be pirated than bought through G2A should tell you all you need.
> The site said it would also start offering developers refunds worth 10 times the sum lost if they could prove keys had been sold on G2A after being bought with stolen credit card details.
The developer interviewed in the original article claims that doesn't matter because the stolen keys are being bought on Steam, not on the developer's own sites:
> Mr Rose responded saying most developers relied on sales via Steam and other third-party stores rather than their own sites, so G2A's refund offer was a "red herring".
But again, shouldn't Steam be the one dealing with the problem in that case? Why do the developers care?
What is it, specifically, that devs are having to do when a fraudulent key gets used? The article doesn't make that very clear.