The UI is separate from the backend regardless. You can swap out Lemmy's frontend with like a dozen others.
Most of Piefed's features could have been implemented with an alternate frontend for Lemmy, but they don't do that because the real primary goal of the project is to coopt and destroy the only corner of the internet where communists have any sense of sovereignty.
Obviously not stated out loud, but I wager the intention is to mirror the Lemmy API closely at first while flooding Lemmy with rabid, astroturfed anti-communist propaganda, convince enough users to migrate to Piefed, then start to introduce API incompatibilities so that interop doesn't work as well for Lemmy users.
I wouldn't be surprised if the project originated as an op in response to a libre, communist led social network appearing.
"fuck I'm sending out bad vibes" 

Who could have foreseen health issues arising when I eat steak drenched in liquid heart attack for every meal?