
i think it also means that natsuka didn't truly love hakushima. he believed he did, but in reality he idolized and glorified him, and he didn't really know much about hakushima's own feelings and what was going on with him (being in love with his brother's fiancee)
natsuka was able to truly love seno, and i think its bc seno was the only one that was really able to understand him (feeling like an outsider/invader in his family). however natsuka was only able to view seno in that way when he believed he was hakushima, bc he was so convinced he could only love hakushima.
but once he realized the person he was really falling for during that time was seno, he was able let go of his feelings for hakushima (it should be noted that natsuka says that he actually starts to like "hakushima" more after the body swap, so he really was falling for seno)
This is so good that i wish that it was longer,but what did hakushima mean when he said "but im not seno" i feel like something's still missing ( ̄へ ̄)