Hagormo would know exactly to what extent the powers he had given them, otherwise he wouldn't have been so confident in their ability to stop his mother and JJ Madara. How would you know Hagormo can't use Rinnegan genjutsu, just because he hasn't shown it doesn't mean he doesn't have it or can't do it. Hagoromo's reaction to Sasuke putting all beasts in a spell was pure astonishment. He wasn't dumbfounded by his ability, he was just praising the extent to which Sasuke could use his newfound ability. Sasuke was gifted the Rinnegan from Hagoromo and the Databook confirms it. He still knew about the technique and that's why he would give it to him under this scenario.
Words aren't enough. Actions are. Seriously though I wish they elaborated more on Hagoromo's backstory in the manga along with his potential.