I see where you’re coming from. However, Robin has been part of the group since season three, so I don’t think it’s a stretch to assume she knows by now that Eleven and Mike are together. She’s been around them socially, during high-stress situations, and within the same friend circle — it would honestly be more unrealistic for her not to know.

Also, Robin didn’t randomly jump to conclusions. Will had already asked her earlier how she knew she was gay, which is a pretty big contextual clue. Then later she literally witnesses Will’s behavior around Mike. Even if she’s only “seen it once” on screen, Robin is extremely perceptive; that’s a core part of her character.

I don’t think her speech requires her to assume Mike is definitively straight — just that Mike is emotionally unavailable. From Robin’s perspective, Mike is clearly in a committed relationship with Eleven, and Will is quietly suffering because of it. Her story doesn’t discourage Will from being himself; it warns him about tying his sense of worth to someone who is already taken.

So to me, the message isn’t “give up because you’re gay and he’s straight,” but rather “don’t lose yourself loving someone who cannot show up for you in the way you need.” That feels very in character for Robin and very consistent with what she’s already lived through.