As someone said above, that's not true. The control line shows the test is working/solution is present. There is nothing in the control line that is dependent on swabbed nostril particles/snot being present. You can try it for yourself, or check out some of the IFU's for the tests. Here is an example:
View attachment 696256
Edit: I am not at all advocating doing this to cheat tests. I think this loophole for self testing is well known too, so I don't think I am contributing to anyone doing this. I am just pointing out why I agree that self-testing is no different than no testing at this point.