I was specifically referring to power, but sorta true. I'm not exactly sure Danzo knew how to counter it though. Naruto at least was canonically receiving training to counter genjutsu from Jiraiya. But failed against Itachi, cuz Itachi.
Naruto's Genjutsu training was more of basic everyday info really. It was already said by Chiyo, that other than not looking into their eyes, tactics when fighting a Uchiha Sharingan Genjutsu user was to - 1)Be sure there's atleast one other person on your side, If not run away, and 2)Fight together, cause If one is put under Genjutsu, the other can disrupt their chakra by touching them.
Naruto's Genjutsu training was Jiraiya teaching him how to exert a bunch of chakra at once to force himself out of Genjutsu.
That seems like a common sense thing to do, that Danzo, who has decades of experience being a Shinobi and knowing Uchiha would know.