Ok, her logic seemed so loopy that I'm trying to interpret the Japanese now. The line for "We aren't dating yet" is "僕らはまだ付き合ってないって", with "yet" being "まだ". If the dictionaries are to be believed, "まだ" can mean something like "yet" but it can also mean something like "still". So he says, "We aren't dating yet (because I'm a coward)", which is the obvious interpretation, but after straining her brain, she chooses to interpret it as "We still aren't dating (despite how it looks)", which I'm guessing is a weird but grammatically valid way of interpreting the phrase in Japanese.
She then does some mental arithmetic something like:
"It really looks like we're dating" + "We still aren't dating" = "Childhood friendship must actually be more intimate than dating! Uncomfortable feelings compartmentalized!"
So basically the same thing she's done every time the concept of romance comes up, just crazier.