In my mind, "cannot overstate" always meant "impossible to overstate", but I think some people interpret/intend "cannot understate" to mean something like "must not understate". I don't know if that's really what they're thinking, but it is how I make sense of it. I have come to just avoid such constructions.

Edit: reminds me of an ancient SNL skit with Ed Asner in which he's a retiring nuclear engineer and as he heads out the door he says to his incompetent co-workers "Just remember, you can't put too much water in a nuclear reactor".