Cumhachd was the one who declared the rivalry, and for her it is very much "love as ownership": certainly for Stultus, quite possibly for Tranquillina as well.
The only problem with that is, Roberto's not a Tytalus. He is a Flambeau, an unlettered barbarian. He is a priori, by definition, incapable of handling the Truth. He doesn't know what the Beloved Rival relationship means. (Actually, as far as I know, no other PC in the covenant has yet, in-character, twigged to the true implications of Stultus calling Cumhachd Mater Cara. Marcus would know, but that's about it.)
If it was MARCUS saying it, Stultus would tear a strip off him, because Marcus should know better. Roberto does not. I see Beloved Rival extending to other Tytali, not to everyone else in the order.
I think Stultus standing up for Cumhachd in this instance would be seen as an insult to Cumhachd. It would imply that Cumhachd is incapable of defending herself; that she requires her student to defend her; that she must be shielded from conflict. (Incidentally, that's precisely why he might do it.)