Your argument makes sense as long as you consider something “private” if it’s shared with Facebook
itself.
You might, if you consider your personal interests to be consistently and permanently aligned with the ToS which best serves shareholder interest of the entity which controls the exploitation of the historical data, and on-going surveillance capability.
The data is pretty much unregulated, and there’s no reason for it to ever be deleted; given it’s size and value it’s likely to live-on much longer than Facebook (the company/legal entity) as we know it today. If they got in financial trouble, the prudent thing would be to flog the lot to a data broker to be resold and repackaged indefinitely.
Facebook themselves have a long established reputation for doing whatever they like and apologizing only if and when they get caught, and doing the minimum possible to ward off regulation or churn.
Just like the use of the term “breach”, there’s a broader use of the term which makes sense given the expectations of a user-base that’s been kept in the dark for years (as opposed to technical people with an understanding of the industry).
For example, my contact list is not “private” if it’s exfiltrated from my phone without my knowledge or consent, to be used for whatever purpose makes most money to whoever controls it (currently Facebook), against a ToS which can change at any time without notice, with almost no legal protection, regulation, or oversight.