I disagree with this line of reasoning. I have seen good/bad examples on either side. I think it actually comes down to someone on the developer team having a high set of standards that they push everyone to subscribe to.
All the open source projects I have personally seen were ones meant to live a long time. When there were code issues, there were always awkward discussions on github about “there should be unit tests here” or “this code makes no sense,” and weeks later the developer announcing a cleanup or some sort. Anecdotal but public scrutiny and pressure is a real thing.
Just as an example, this is why Bitwarden started getting some automated testing - lots of propelled bumping github issues about it in order to get it more visibility
It's only tangentially related to code quality but I do think open/free source is the only way to write sustainable software if your aim is to change the world rather than ones bank account (so to speak).
There's terrible code all over the place, although it is definitely true that no one's going to clean up - even source available - proprietary code out of kindness of their heart.
Just as an example, this is why Bitwarden started getting some automated testing - lots of propelled bumping github issues about it in order to get it more visibility