Making out at a party seems pretty normal to me even if the interactions with her have been minimal. And sure it's strange but we can also reject the idea of making out with her I just believe there are far too many if's and but's to say it's Riona yet.
You could boil the plot of almost every movie down to "ifs" and "buts" because they're invariably about a bunch of random occurrences that inexplicably line up perfectly for the story. So many plots are all about characters being in the right, or wrong, place and the right or wrong times and getting swept up in the story, whether they like it or not.
Sure, Riona might have just been doing it on the spur of the moment and she says that's what it is, but can we be sure she's telling the truth? At the start of the episode Quinn may have been pointing at the MC as her "2nd move" on her metaphorical chess board, so this would surely help with that as, if found out, Riona could say the MC put her up to it and since people saw them looking quite friendly with each other after the Cluck was sent out, it doesn't look good for the MC.
Yes, Riona couldn't have known the MC would show up outside the library after the Cluck was sent out, or that he would speak to her, but when he did she probably saw an opportunity to help Quinn's 2nd move by making her own move and whether you kiss her or not, both outcomes could still potentially be spun against the MC. Hence, a totally random moment full of "ifs" and "buts" that may be more important than it seems.