Regardless of whether that is true or not FB had to do this sooner rather than later or risk losing the ability to enforce the license in the future.

They can’t lose the ability to enforce their license, they could leave it alone until the day before the copyright expires and launch a bunch of lawsuits if they wanted to.

Assuming that they can even copyright a bunch of model weights.

Patents, on the other hand, need to be protected or they can be lost.