Yes, but the property here is control over the legal rights of the code, not the Github account.

Having copyright over the code means being able to provide copies under a license of their choice. They still wouldn't be able to do that: relicensing the code would be illegal. They could only use it under the same license they already had.

The Github account is not IP, it's just a property of Github's system, and it belongs to them (GH), not to the copyright holder.