@Riri Since his memory loss is different from the usual complete memory loss cliche, I would say it's something like this: Since he only lost his memory for the past 2 months, they both used that to say that they're going out with him, from around that time frame, and his brain using both his ideal of "Happiness is to be loved" and the information he got from the two, is creating fake memories to explain why they're going out, at least for Ruka right now
And when he said about her confession on a flower field, she made that face, because that never happened, so she was gotten by surprised, and had to fake her way out by agreeing with him