It's not a harem ending he already said this. It's still harem because all five of them like our boi Fuu. This is just Fuu finally coming to terms that he has to choose one, and I can see why this is going south. Because now all five of them are going to try their hardest to fight for his affection so that's probably why it says at the end page
" didn't end well "
This is going to be the "fighting arc" this arc is going to be all the sisters trying to fight each other so they can take Fuu for themselves, prepare for war boys.