>a classic failure of trust boundary definition - they effectively created a system where client attestation was accepted

Can you elaborate? I'm unsure what a trust boundary definition means in this context and how it relates to attestation.

trust boundary basically means a spot in execution where the trust level of code changes (aka privilege level) and thus needs reverification

It means you trust something with lower trustworthiness without (re)validating, or even trusting it at all if the validation isn't all but guaranteed. The boundary is when you switch between levels of trust.

Trusting something outside of your control is a good example. When your trusted game server trusts the untrusted game client when it says "trust me, it was a headshot" without validating this.

Fortunately they have a solution for trusting untrusted clients already! They just need to run an anti-cheat for their anti-cheat.