Hmm... I actually like this change (retcon?) establishing that there was more of a basis of physical intimacy to their relationship than we readers might have assumed... But I still just wish this extra was created closer to when it takes place chronologically. That's just because I'm really picky and "perceptible lack of a pre-planned plot" is my biggest fear when reading ongoing manga though lol.
ANYWAY this dovetails really nicely with the emerging arc from volume 7; my reading was that Shou is unnerved by her own feelings of desire that spark when they touch each other, not simply reacting to an unfamiliar situation. This extra giving Umi a little uncertainty on her side too suggests that the story could be heading towards the sort of strong emotional climax I would like, I hope!