I agree with this assessment but for many applications it's a viable approach, until the attacker goes off and writes their own shader to solve the PoW. We go to back to threat modeling here, and looking at the amount of effort vs gain here.

They're now integrating Argon2ID in an attempt to squash GPU hacks but it places ridiculous demands on the client being Memory hard.

This seems like the real mechanism here is not actually proof-of-work so much as security-through-obscurity.