Oh, it's not out of the blue at all, it was foreshadowed constantly. Karkat's conversation with Jade, both of Nepeta's shipping walls, etc. Hussie throws some curveballs sometimes. This was not one of them.

I just don't subscribe to the way of thinking the moirail automatically equals greater than or equal to best friend, even if it usually does, like with Equius<>Nepeta.