And if the "attester" decides that IceWeasel on Ubuntu (or Firefox with uBlock/uMatrix/NoScript) isn't "trustworthy," but (unmodified) Chrome is "trustworthy," you've just created vendor lock-in.
The same proposal suggests that users who fail the attestation still access the content. Which is apparently how the Apple version of this same protocol already works.
That's not a "nothingburger" IMHO.