People could fix each other’s intentionally introduced bugs and make a living that way.

The argument is less convincing when the bugs are a couple years old. There could be an exemption for that, but it’s also more work to verify (Git histories can be fabricated).

They could anyway, one person intentionally introduced bugs and the other reports them. The reporter just avoids ever contributing code themself.

But doing any of this repeatedly without getting caught seems hard.