Quite the opposite. Synapse was licenced under Apache licence, which allowed everybody to make proprietary fork.
Now only Element can do proprietary fork, as they are the copyright owners and owner of the work can relicence the software as they want.
The CLA that Element require to contribute changes to their Synapse version is the controversial thing. Because if you contribute, the lines of code you made are your copyright, so if Element takes at least one community contribution the would be locked to AGPL. What CLA does is that you sign off those rights.