That's correct, that rule was not in the rulebook that shipped with the base set, but was added in their Revised Rulebook which included several rules changes and clarifications, the most significant of which were the timing clarifications for React windows.
These rulebook updates were pretty highly publicized within the community so I'm very surprised, shocked even, to learn that PBE completely missed them or were otherwise not aware of their existence.
EDIT: Removed nonconstructive snark