Yeah, I'm curious about this as well since I'm going to clean flash A13 now and I used Swift to backup my most important apps. I'm still on A11 on my Pixel 5 and skipped A12 on purpose, but because of the great user feedback on the speed and stability of A13 I'm going to update now.
I will report back on how it went, but somehow I really can't imagine why it isn't possible, I mean after all you have root acccess ...?
Update: Sadly it seems to be true. I can restore the apps, but even though it restores seemlessly (at least in Swift), it seems like the user data cannot be restored, as apps always launch like freshly installed without any existing data.
Edit: Well guys, it's weird. It looks like restoring app data with Swift does work, but for some apps it works, and for some apps it doesn't properly restore the data. E.g. Google authenticator doesn't work (launches with introduction message screen), Card Emulator Pro or AdAway restored flawlessly (with all data).