This is true - people who have impaired cognition will believe that their judgement is fine while it in reality isnt. It's either worse than normal or suboptimal at least. In no way, though, is it average or above average.
Drinking alcohol can impair both your motor skills and your cognitive function. However, people regain the former much faster than the latter. This false sense of recovery can lead people with impaired judgement to believe that their cognitive functions are normal, which can be very dangerous for themselves and for the people around them. In particular, this is a common cause for road accidents.
