I think it may accept both the underscore and hyphen and just normalizes it.

This is correct, it does accept both, and the help lists it as underscores not hyphens. So the comment about it ought to be hyphens not underscores is incorrect. Either is fine.

When I went digging through the code for v8 I believe I saw examples of it both ways. I'm not dead sure though because I don't know very much C. I just checked because I was hoping they weren't actually using an invalid flag for all that time.