I think it's slightly rushed in the webtoon cos it misses a lot of the internal monologue where J identifies LSY as exceedingly beautiful, but if you read the novel it's very clear that at this point they care for reach other regardless of knowing each other's identities or not. There's also hints in the webtoon of LSY randomly touching J way before he even suspects CEJ is J if you go back and reread

haha look at my babies flirting while in a wold ending situation