It isn't. Look at the words you quote: "Then we bill the runtime fee based on all new installs that occur after January 1, 2024." The new installs are new instances of the Unity runtime.
I'm really sorry they are screwing you but words have meanings, and this is not retroactive. One legal scholar writes:
"A retroactive statute is one that operates as of a time prior to its enactment. A retrospective statute is one that operates for the future only. It is prospective, but it imposes new results in respect of a past event. A retroactive statute operates backwards. A retrospective statute operates forwards, but it looks backwards in that it attaches new consequences for the future to an event that took place before the statute was enacted."
And of course retroactive and retrospective changes cannot be made to contracts (vs law) without the agreement of parties, or a court decision. Unity is saying you have already agreed to these terms for existing licences, and must agree to new terms for new runtime licences. Something you can dispute in court, but not without significant financial risk.
