they wouldn't need to precalculate anything, they have a real-time probe based GI system. they simply didn't make it look good in its current form because it's not used for closup lighting.
they could probably modify it to be closer in quality to Cryengine's SVOGI, and thereby get an ok looking GI solution without any prebaking.
but they also could bake some of the more important lights in indoor areas as well. that's not a big deal to do, at least if their engine is somewhat decent (which it is, based on previous releases).
like in UE5 you can just use your Lumen setup to bake the lighting. meaning it will look exactly the way you saw it in the editor.
the guy who made the fan remake of Toy Story 2 in UE5 did that. he used Lumen before, but it looked bad and ran like ass, so in the last step he baked the Lumen GI, making the game look far better and I think increasing the framerate by like 3x~4x or something.
so if your GI is already set up the way the artists wanted it to be, baking it when needed shouldn't be an issue.
again, they wouldn't need to do it for every light in the game, just for the important indoor parts, and then just use their probe based GI for the rest of the game.
honestly, this is how I hope the rumored Expedition 33 port does it. that would honestly be amazing.
like, no joke, if they bake the GI in the Switch 2 port it could turn out to be the best version of the game

.