they track everything that they track from normal browser tabs. it's just that they can't access any cookies that they may have set in your browser elsewhere, and any cookies they set after the tab is opened will be lost when the tab is closed again (or maybe only when all incognito tabs are closed) and anything you did will not be added to your own browser history.
it may make sense to compare incognito browsing to someone with memory loss. you will forget that you talked to me, but i'll still remember (your ip address and your browser fingerprint, and that fingerprint may be enough to link your incognito tab to your other tabs and to your actual identity).
Google websites don’t know whether they’re being visited from an incognito tab or not, and Chromium works hard to ensure they can’t know (because knowing a user is in incognito mode is itself a privacy leak), so they track anything they could otherwise track without the logins and cookies which are excluded from the incognito sandbox.
I searched around, but Google's explanation isn't transparent and the stackexchange post didn't reveal anything of consequence.