For the sake of the argument I'll ignore the fact, that this won't really happen.
If Reddit were to 100% implement ActivityPub (the standard behind federation) and actually play along like any other platforms (lemmy.world, kbin.social, etc.), then from the network perspective it would be just like any other instance.
It would have the same trust assumptions as before - except that it nicely interfaces with other instances.
Interestingly, this would make both Lemmy and Reddit more valuable, as people can comment on each others communities without having to switch apps/websites.
Communities were already somewhat fragmented on Reddit, and it would be just more in the new place.
Obviously, Reddit would be the odd one with being like 1000x or so bigger than the rest.