If his role is important, this should read, "his role cannot be overstated."

Just like people they don't care at all should be saying "I couldn't care less."

It annoys me, too!

I would argue that overstated would be wrong because you could surely overstate his role. "He was the only reason Tesla existed, would be overstating his role.

I think "his role shouldn't be understated" would be the correct phrase given the context.

I agree with you. Can I blame "first thing in the morning" posting on my error? Either way, I was incorrect and you have corrected me.

It seems to mean that any statement that is relatively correct will have to detail the amazing performance of the person in question, exactly because it was so essential.

It was so good that no one can even try to badmouth it. (So it's a hyperbole of course.)

They did some terrible things to our language after appropriating it but i'll give this one a pass.

"Its impossible to understate the importance of his role"