The point by @iabarry is that some (perhaps most?) board fabricators DO NOT want the board outline shown in any Gerber file except the actual Edge.Cuts file. (Don’t ask me how they ensure layer-to-layer registration, since I haven’t seen a “target” or “bullseye” on a PCB layout for a decade or more.) The error message may not be squawking about the Edge.Cuts layer, but rather complaining that Edge.Cuts information was found on other layers.
The CAM tools aggregate all the different layouts and place it into a super layout which will have those marks for their processes, no?
The only thing not covered by that is for pick&place machines, once they assemble the boards, which is why one needs some markers for that.
But that’s just me guessing really