The claim that killing 3rd party cookies is anti-competitive is some Marketing Company Bullshit. A group in France is also suing Apple on antitrust grounds because they're going to ask users if they want to be tracked.

What's actually anti-competitive is the idea that Google will kill them off and replace it with a solution which they control, forcing advertisers to deal with them and their ad products directly in order to do anything.

Whether that's actually what's happening is another question entirely.