i too, was questioning this sentence... in the end i think they mean lover as someone he loved but not in a romantic way, like familiar love.
It wouldn't make sense otherwise, the narrator can't spoiler things like this damn ヽ(`Д´)ﾉ
In the previous chapters around the time when the prince confessed to Shuli, it was insinuated that he had feelings for her but he knew he had to repress them. Jeremys fiancé mentioned it as well. She said something along the lines that she had to nip it in the bud before Jeremy realized his true feeling for her.
and maybe even if he didn't acknowledge this feeling as something romantic after mc died, people could have interpreted his reaction as such. that phrase was a narration through nora's pov so maybe that's what he heard or the impression he got of what happened to jeremy
"Neuchwanstein went half mad longing for his dead lover" what does this mean?? did Jeremy love her but not as his family?