They certainly see it as a big enough grievance to include in a manifesto type document which repugnantly entitled.
You seem to suggestthat OP's control over their own site content (ie the manifesto) is somehow an affront to your belief in google's free speech.
You certainly see it as a big enough grievance to write a comment on HN which is repugnantly entitled.
They get to pass comment on google's behaviour (a comment which, incidentaly, a lot of people seem to agree with). You get to pass comment on their behaviour. I get to pass comment on your behaviour.
Holding people and companies to account in a soft manner, without resorting to force (legislation counts as force) is an important aspect of a functioning free society. The freedom of speech is not freedom from criticism but freedom to criticise.
I don't think the letter authors were calling for an Act of Congress. My read is that they were calling attention to the damage that AMP does to the content providers. And perhaps they could have put more emphasis on the fact that it's in Google's long-term interest not to bleed content providers dry.