Users are bound by the version of the terms they agree with when they start using the product. There may be a term that says ongoing usage when the terms change constitutes acceptance of a change.
Unity are trying to say they can make the change retroactively, but the 2022 (and prior) terms apparently included a clause saying that if future changes were detrimental to the user they could stay on old versions of the software and remain bound by the old terms. That’s one angle Devs could use to tell them to get fucked There may be others.