with several high profile cases in the linux kernel i think this belief has been shown to be overcautious and maybe this has led to a relaxation of the position.
The idea of assignment to FSF is not merely to guarantee to the world that the original copyright holder won't offer competing licenses or something.
That seems like it would be potentially incredibly disruptive (e.g. if a contributor decides several years after a lot of work has been built on top of the contributed code to claw back the rights), especially because as far as I'm aware there isn't a separate license you give the FSF on a contribution, just the assignment. AFAICT it wouldn't even make sense to give the FSF a license because you don't own the code anymore, the FSF does.
I can't find anything on the FSF website that allows this.