I spent some time working on a RN project, and even though the whole idea of declarative interface, state update, etc is interesting, it is not exclusive of RN - and there is the whole rest of the story: RN libraries are a mess and usually buggy, there is virtually very little compatibility between native and web, and.. have anyone ever tried updating a RN codebase?
I remember we had to recreate the full project and copy the files manually because everything was broken beyond any hope, this after spending a full week trying to fix the build.
For Shopify seem that even a PWA could fit, and would be much more compatible, reliable, etc.
By the way, one of their justification is "in-house know-how", which I heard so many times before... This is usually how you justify crappy decisions by using inferior technology and then spending 2x the effort to try to fix it. I hope it doesn't happen with them, but it does like like that's the way they are heading to.