I think in this case, she wanted to get closure. I would've thought that she waz trying to compete by doing what she did, but I guess it was a combination of factors:
Good friends + didn't consider Tatsumi to be the "special someone" + lack of boundaries = being close without awareness. Although, in a normal situation where she acts the way she did at someone else's house (and saying what she did), anyone would've thought she still liked him.
Maybe she still did. She felt bad because she genuinely liked Subaru and grew an attachment to him (friends to lovers), but felt lacking. She didn't want to give up their friendship, but the way their relationship ended left a lingering smolder for her.
He felt guilty because he was still unsure of everything when he dated her. Interested in Tatsumi, but confused as to what he was feeling and thinking; and why. Upon seeing that someone had affection for him, desspite having unsure feelings, he still tried to entertain this relationship. Maybe he, too, genuinely tried to like her and categorize his feelings for both of them.
Hence, the only favor he could do for her is agree with her terms; plus, they still liked each other - just not in a romantic way.
Unfortunately, with Subaru being unable to tell his ex in time that the guy he's with is the one he was currently seeing, it just caused tension and insecurity.
All the disturbing stuff aside, I’ll never understand exes turning into friends and then wanting to meet the actual boyfriend or girlfriend. Then, getting all touchy-feely in the kitchen at their lover’s house? I don’t know if people are just that inconsiderate or if I’m just too sensitive to this kind of thing. And it wasn’t even the seme who mentioned it was his ex—like, seriously?