That's false: minors can enter a contract with parental permission, and some of the coverage has Facebook saying they had parental permission for all underage participants.

But further, even if it was a violation if minors were involved, that'd leave open the question of whether use by contracted adults was compliant under the terms. (And supposedly the Google app wasn't offered to minors.)

And, paid research subjects meet the legal definition of contractor, as outlined here or elsewhere:

https://dictionary.law.com/Default.aspx?selected=939

Simply insisting "definitely not true" is not convincing.