A separate version is "open core", not "dual license" - that's a different business model.

Use case differentiation by means of providing the same thing under different licenses is what dual license (or multi license) means. One license (say, GPL) will be appropriate to some use cases (downstream open source projects) but not for other use cases (downstream proprietary software) and so the latter has to pay for a license other than the GPL.

> I didn't find any explicit mention of copyright transfer

It's possible they know the people who've submitted code outside of github and handled it out of band, or that github has some way I've missed of requiring submissions to assign copyright in a way that wouldn't be visible to us. It's also possible they're being legally reckless and might be subjecting their paying customers to potential (if perhaps unlikely) copyright claims from their submitters. There is nothing about it being the same code under both licenses that would imply they wouldn't need their submitters' blessing in some form to sell their code under the proprietary license, and the more implicit blessing the more likely it'll wind up in court at some point.