Since he’s the one that is imagining her get fucked he probably assumed she wanted to do a threesome with another male instead of another female. His perverted mind immediately went to a Mitachurl. She was thinking about a Mitachurl too, but she really wants to see Aether getting fucked instead. They are sorta on the same page but not 100%. Either way both fantasies can be easily achieved by simple communication.
The fun fact here is that in Nyantcha's previous aether and keqing fanarts includes mostly couple behavior that go against anything implying someone else, take the one that was childe about to literally rape Keqing and then aether chops Tartaglia's head off with Keqing's mistsplitter, or the one where aether apparently apologizes to Keqing about his actions after a Mitachurl gave him his own medicine. Which makes this one out of place, not to mention that there's another ones where keqing is also the one that fantasizes about aether only for herself and so on.
