Edit: Thanks for your kind comments on disabling contributions. I'm aware of the possibility, this is just a meme.
Contributions in open source projects are great, needed and make the world a better place after all. Therefore they should be declined politely, optionally by giving a reason.
I hereby inform you that I will no longer accept contributions to this post /s.
Sure, but that's not what the meme implies.
Verifying that the code doesn't contain regressions, bugs, or vulnerabilities, that it doesn't conflict with whatever the owner is actively developing privately, in addition to making sure it wasn't vomited out by a goddamn clanker, is a huge burden on a solo developer. They are free to decide whether to take on this responsibility.