Nah, I'd say that the sign is irrelevant - it's an absolute value scale, which is fine. I mean, very negatively impressive is still impressive - it has an impression

.
His scale was defined with 100% being perfectly
IMpressive, and 0% being perfectly
UNimpressive. 50% is essentially neutral, although he referred to it as "unimpressive", and I could see an argument that way, although it's really splitting hairs.
It's kind of like arguing over whether to classify zero as being neutral or being "non-positive". Both are true, it's a useless distinction with no practical value outside of a few very specific cases.
So I'm fine with 50% being labeled "unimpressive".