I've heard that because this isn't a numbered release, it's not (necessarily) a LRG exclusive release. So it's very well possible this will be sold at other major retailers (for those that don't want to do business with LRG).
There's simply no way of knowing fir 100% guarantee if newly released Switch 1 games will be fully compatible with Switch 2, not at this point at least.
Hopefully Nintendo has some kind of submission program for devs moving forward to guarantee backwards compatibility before release.
I don't enjoy some of the stuff that goes on with LRG, but at least they eventually ship their games unlike SLG. They are the current scam kings of physical games.