There's no reason to even have a placeholder in the Play Store, since app names don't have to be unique (the unique part is the package, which in Java uses a reversed domain format to prevent collisions -- com.rubenmayayo.lemmy in this case).
Doesn't take that much work to implement the Lemmy API enough to get it fetching a few posts for a screenshot, but doing it for everything Boost can do will take quite a bit of effort which is exactly why he started the preregistration - if enough people show interest, it's worth doing.