Probably a reporting error, since there's no stigma against having mental illness on the far left like there is on the right.

Running the risk of being pedantic, that wouldn't be a reporting error, that would be overreaching conclusions.

I'd guess it's just because they're more open about it, whereas people on the right are the opposite about it.

Could be an explanation, however I don't see it supported by any data so it can't be stated with any certainty.

I don't think you can jump to the conclusion that "leftists" have more mental illness overall based on self-reporting data...

This is actually a very good observation, self-reporting can be tricky, I remember some years ago an analysis of the subject, and there was a study that indicated that men reported having on average 3 times more sex then women, which math doesn't add up.

I also recall reading something on how most evaluated their driving skills, there was a tendency of people reporting they were above average or higher. So, the average was above average. ¯\_(ツ)_/¯

In this case the premise was: "The more left wing people are the more likely they are to have been diagnosed with mental illness." , a possible explanation (like Tim Pool stated at 4:20 ) is that left leaning people could be more likely to seek diagnostic/help. You can't get diagnosed with something if you don't go to the doctor. This kinda reminds me of the survivor bias post I did a while back.