It's circumstantial evidence. Alot of people who aren't smart get confused and think circumstantial evidence isn't evidence... and they're wrong. For instance, if I got outside and see snow all over the city, I may not have actually seen it snow, but I can feel very confident it snowed. That is only circumstantial evidence, but it's none the worse for that. We have only circumstantial evidence for the awesomeness of Hashirama... but it's very good evidence, so we can feel confident he was indeed at the top of the Shinobi tiers with a handful of others like Madara.
The circumstances of the Mizukage's fight make it very clear he can attack people during the genjutsu, even while a long distance away... we know he can, because we can see scans showing he did it. How he did it doesn't even matter to be honest, only that he can do it, and do it with enough force that Gaara's ultimate defence is crumbling against the force of his attacks. Your rebuttal to this evidence of "I didn't see it on panel" is a weak response. We don't see alot of stuff on panel... we never saw the Sage do anything on panel... but that doesn't mean there isn't evidence of what he can do.
