That the ruler is related to the antenna tuning is the initial guess of the author too, but he is unsure.
But come to think of it, why is the PCB silk-screened in the first place? I don't think such a cheap board will ever get serviced by the manufacturer, and slikscreening is cheap but not free.
My guess is that it is "just in case". If something goes wrong, the silkscreen is there so that things can be fixed by an assembly line worker. And they identified incorrect antenna dimensions as a potential problem and left a ruler there. Turned out everything worked fine and the ruler wasn't necessary.
Virtually every board is going to need some kind of marking, so if you're doing a silk screen anyways it almost free to add more stuff to that silk screen.
I do software quality assurance, and it wouldn't surprise me if this was put in by request of their QA department. I don't know if a human looks at every PCB, but however many they sample to QA, this would be a quick way to check an important feature of the hardware.
I'll go with occam's razor and say it was simply not removed from the development board to the full production board.
It's probably related to tuning but on a production board? Normally if you need it as part of your production process it would just be part of the tuning tool itself.
But honestly I cannot see how it would be used in production since the antenna length is determined by that point.
But come to think of it, why is the PCB silk-screened in the first place? I don't think such a cheap board will ever get serviced by the manufacturer, and slikscreening is cheap but not free.
My guess is that it is "just in case". If something goes wrong, the silkscreen is there so that things can be fixed by an assembly line worker. And they identified incorrect antenna dimensions as a potential problem and left a ruler there. Turned out everything worked fine and the ruler wasn't necessary.