The rules aren't that simple. The combination should be considered holistically against antitrust law. It's a judgement call that ultimately comes down to a judge or jury.

Antitrust is also federal and is probably generally supreme over state law. E.g. even if Google fairly acquires talent according to state law, it can still be determined to violate antitrust.

This poaching scenario (where Big Co pays above market to a large number of employees to drive a competitor out of business) is very similar to dumping (selling goods below market etc.), which has been illegal for a long time even though "selling goods" is generally legal.

However, since the government (DoJ, FTC, etc.) is the ideal litigant, you actually need someone who works there to not get fired for long enough to complete investigation and enforcement. In a lot of anti-trust cases, all the direct participants benefit: the employees get paid more, Big Co consolidates power, and Small Co gets paid off. The "losers" are most distant like their customers or uninvolved competitors who aren't in a good position to sue.

You can ban a compound action. We have antitrust laws.

Which is why at the top end we are supposed to have complementary laws. Bans on non-compete increase competition at the low end. Antitrust and government regulation/licensing at the high end prevent monopiles and thereby increase competition. The problem today is that we are dropping the ball on the high end, letting the Googles stamp down competition when, in decades past, they would have been broken up.