Apple is giving up a lot of money, likely hundreds of millions, by not letting Fortnite back in.

You could argue that in the long term Apple might be calculating that the fear effect on other companies might pay that back, but that's highly arguable. It's unarguable they are giving up a serious amount of reliable income to make this point.

Maybe tens of millions. Certainly a lot to us, not so much to Apple though I'm sure they want the money too (hence the whole lawsuit thing).

Epic was ordered to pay $6mm from the case that they lost which was sales from pre-installed apps over around a 3 month period [1]

  [1] https://www.macrumors.com/2021/09/13/epic-games-pays-apple-6-million/

You are missing the wood for the trees here: Apple may give up some money for the Fortnite game, but meanwhile they get to hold on to the money they rake in on other parties' products. If they accepted the Fortnite change then they would lose that other money, which absolutely dwarfs the money made on Fortnite.

I'm a bit surprised that this needs to be explained.

Apple won the case, they don’t have to accept the Fortnite changes. In fact the court ordered Epic to follow the rules, if Fortnite goes back into the store.

How would letting Fortnite back in the store, and taking their money, reduce their revenues from other developers? You’re not making any sense.

Ehh that is generous to the Fortnite fad. It's definitely a fraction of the memetic giant it was even when this litigation began.

I would postulate that 80%+ of Fortnite revenue is now from the item shop. In which case apple is "sacrificing" 20% to try getting the 80%.

On the contrary, I think it is more about making an example out of Epic about the severe consequences of litigating against apple.