Wouldn’t be the FAs disregarding a manual on their own authority. Should be the Captains call one way or another, once the FAs (rightfully) called the cockpit to inform them of the situation, and the cockpit crew checked it out, thus placing the decision on the Captain as the approval authority. Good CRM and all. The RO had to be sent back to do the job anyway, it seems.
It’s not like the FAs would’ve been opening the door with zero permission as if it was any normal circumstance and simply disregarding the flashing light. I’m sure (I hope) there was some coordination and permission to open the door from the Capt, of the fact the pressure diff was confirmed to be zero and there’s a hole in the fuselage in the form of an open cockpit window, with free flowing air going back and forth into the cabin. A light that is supposed to be showing a pressure differential problem, when there’s obviously no pressure diff at all and confirmed as such by the cockpit crew. I highly doubt the Captain would clear them to open a door that is in any way potentially unsafe to open.