No if you declare war on a federation, you can take planets to anyone in the Federation.

I made it that you can not take planets or vassalize or anything to other members of an Alliance though.

This is mostly because otherwise you can attack and get demands for one empire multiple times even if you have a peace treaty with them.

And I let it happen for Federation because since they feel much more as one big entity.

"The attacker can not have war goal targeted to the ally of the defender" Does that mean if I declare war ona a Federation, I can only take planets from one empire?

nice