I have a 2016 F150, which I think should be the same as yours. When you put the truck into flat tow mode, there is a dashboard indication that tells you that flat tow mode is enabled. There is no steering lock, and the ignition is off and not in accessory mode. I have just towed my F150 over a thousand miles and can confirm that it does not rack up on the odometer.
BTW, the flat tow mode is very well done on this truck. It requires a deliberate action to engage, so there's no chance of being accidentally done. To disengage, it's as simple as putting the shifter back into Park and starting the engine. You'll hear a "thunk" as the electronic transfer case switches out of neutral. And all the while the truck is in flat tow mode, every time you open the driver's door there is an audible chime and dashboard warning.
Barry & Gigi (and our 2 cats)
2017 Newmar Dutch Star 4369 (2016 Entegra Aspire 42RBQ)
2016 Ford F150 Platinum w/'14 Ultra Ltd on LoadAll ramp