I wonder if people signing up to that waiting list will start seeing a lot more adverts for headphones across YouTube and Google Search. I'd wager entering an email address on the site doesn't just put you on a mailing list for Pixel Buds.
Pretty sure that'd be in violation of things like GDPR and risk netting them fines. Also, why would they add you to a list of "people interested in other brand headphones" when you sign up because you're interested in theirs?
Google already does targeted advertising and operates under GDPR.
Using personal data given to them for purpose A for purpose B is frowned upon. It's not just about targeted advertising.