On this - They will have announced this before going in the the Holiday season so people don't hang back on buying Switch games, as they know they'll work on Switch 2. If it was digital only, they wouldn't have even announced this as it would be a mildly new feature that a smaller percentage of users would actually be able to use.I don't know on what basis people have decided BC doesn't extend to physical copies? Is it because they announced BC and continuation of Switch Online at the same time?
I don't think there's anything more recent than that comment of any substance, so not sure why people have decided it's physical only.
If anything, the sentence 'its compatibility with Nintendo Switch' hints at potentially even supporting inter-generation backwards compatibility - i.e. if you played MK8 on your Switch 2, you could play online with people playing on Switch 1 still.
I'm fully expecting all Switch 1 games to work in "docked" mode on the switch 2 in handheld state. (i.e. 1080p rather than 720p if the game allows.)