If you are working at a place that does OSS that puts you miles ahead of anyone working on closed source systems.
Additionally the legality of OSS commits is technically still kind of grey for a lot of people with generic "your code belongs to the company" in their contract with their employer.
Yes, if an employer can easily determine you know what you are doing, you are miles ahead of people who are unknown quantities.
See also filters like college degrees and past experience, both of which also provide evidence you know what you are doing and put you miles ahead of people without them. Should we also not look at past experience and education?
I am speaking from a requirement standpoint. By requiring OSS commits, I am heavily favoring people who worked for OSS companies. This may be a good thing or a bad thing.
I do not disagree on the value of looking at OSS, I just feel that requiring it is going to cause you to pre-emptively filter your candidate list.
Now if that is what you want there is nothing wrong with that, it just may not be obvious that is what you are doing.
Note that I am assuming you are looking for complex work, github is great for seeing if they can at least program, but I would trust a quick whiteboard problem more for that level of technical aptitude.
Additionally the legality of OSS commits is technically still kind of grey for a lot of people with generic "your code belongs to the company" in their contract with their employer.