Interestingly absent is Google.

Google only has visibility into what you're doing as a subcategory of "websites you visit", so I'm not clear on the confusion here.

The confusion is that google makes chrome. Plain and simple. A feature (incognito) with a bypass (analytics) is a non-feature since they are both done by the same company. So the practical solution is for them to be not done by the same company.

What's the legal basis for this claim?

If you start from the axiom that Google should be broken up, this is a reasonable conclusion. But punishing a company for keeping separate products separate would actually undermine most anti-monopoly law.

They are not separate in the monopoly sense though, are they ? Chrome, presumably a cost center, exists only to facilitate google's other businesses. Nobody has punished google for keeping products separate. Quite the contrary. It has always been about extending dominance in one segment to an unrelated one. So google could say that they treat all trackers equally in the incognito mode, and that would be correct, but if google analytics is the revenue center, and the cost center is giving it a free pass, the argument that other trackers can also bypass incognito seems weak.

How is combining tracking data keeping the products separate?

What tracking data is being "combined"? I see nothing suggesting that data from chrome is combined with anything else.