> ...is also completely unenforceable...

It's exactly as enforceable as when projects require you to swear that you can legally assign copyright of your submissions to the entity that runs the project, and/or swear that your submissions don't infringe on anyone's patents, and/or -as the ReactOS project does- swear that you've never seen the code that the project is reimplementing.

If someone swears that they're conforming to the rules and you later find out that they lied, then you ban them forever, publicly announce why, and -if needed- purge their contributions and do your best to alert affected users about the tainted versions. These sorts of policies work for the same reason ordinary doors and door locks work: most folks aren't profoundly antisocial shitbags.