Is there any legit positive impact of LEGO on mental health?
Yes.

Here is a study that demonstrates one way to use LEGO in ADHD counseling.

If you are looking for a more rigorous outcomes-focused methodology, there's another study that looked at the impact of LEGO on social skills development in children. It found significant improvement compared to the control in a pre-test, post-test study.

If your interested in the developmental side of LEGO, here's a study that shows a correlation between complex LEGO play in 3 and 4 year olds and their secondary school mathematics achievement.

On the adult side, here's a study that shows a decrease in workplace stress after a group LEGO activity.

I'm sure there are loads more, but that's what a quick search on Google Scholar turned up.