Bitcoin is only resilient to fraud only if you define fraud as "violating the rules of the blockchain". Any definition of fraud that involves meatspace is possible.
I don't see how that is even remotely possible with Crypto. How did Steam lose money ? My guess is they might be accepting it through some payment provider (so not a true crypto) who has in their T&C to be able to reverse transactions (which must be happening in Cash on the Steam facing side).
Seems like it was multiple things and "fraud" is just a catch-all term and Gabe didn't bother to list everything that they considered to be fraud.
Criminal creates a shit game and then uses the crypto to buy that game (with alt accounts) and since *very* few developers would be happy with getting paid in crypto, Valve has to convert it into fiat(?) currency.
Because they don't want to process illegal transactions with illegally acquired money, because they don't want to abide or facilitate criminal conduct, wherever possible. Life and business is not merely nor always about the potential to lose money through legal ramifications.