Their contract terms for new authors are set in stone, atrocious even for the publishing industry, and unavoidable for most because they have a stranglehold on the ebookmarket.

Honestly one could go on and on and on about the shit they've pulled/tried to pull at various times.

Anyway... Why would the hypothetical e-publisher need to be on the fediverse?

To be honest right now that just seems like a recipe for getting no publicity and no sales, which no author who wants to actually have an income would do just to promote fediverse. There'd have to be an advantage beyond already existing platforms.