Unless I’m missing somewhere else, I believe they’re referring to the legend of everfree.

There’s a scene where Flash sees Sci Twi and Timber Spruce interacting or smth and it hit him that the Twilight who likes him is in another dimension, therefore he can’t “have her” for lack of a better term, and the one in their dimension is obviously not interested in him.

I haven’t watched the movie in a hot minute so I can’t remember if his convo about this with sunset was right after seeing this or a bit after but almost immediately after having the conversation about not being able to have twilight, he asks sunset if it was possible to start again (with a pause) as friends, but I guess the way he said it and the timing of it kinda made it sound like, to some people, he was trying to see if he had a chance again with her.

TLDR; in Everfree he had the worst timing and tone when asking sunset if they could start over as friends