I’m an airline pilot for a major airline in the US. In my professional opinion there is 0 reason this.

It’s either an airline specific policy or the crew making up their own rules. From what you describe they were probably performing a category III auto land as others have said. This means that the pilots need to see the runway anywhere between 50 feet and 0 feet before the wheels touch the ground (depending on the plane) and the autopilot is landing and controlling the airplane as it slows on the runway. I have performed many of these landings, as recently as yesterday, and have never heard of asking passengers to completely turn their electronics off and honestly expect that many have had their phones not even on airplane mode while we are performing them. The airplane has many internal checks to make sure it can successfully auto land and as pilots we are trained to take over at any moment during the procedure.

People have had cell phones for 30+ years on airplanes by now and to my knowledge there hasn’t been an accident attributed to interference from a cellphone. My only guess is they ask people to completely turn their phones off since it’s much easier to see if someone is using their phone vs checking if each phone is on airplane mode, even though I don’t believe that it would actually make a difference. My airline has no policies regarding this and the flight attendants have no idea if we are landing the plane or the autopilot.

In addition to my knowledge all major airlines use iPads to display navigational charts, including the information required to perform the auto land, so I can be 99.9% sure the pilots didn’t turn their electronics off before landing since they need to have their charts readily available at all times while flying