Here, the author of the quote is taking into account the shutter speed 1/50 that you have to set the M6 to in order to trigger the flash from the SF20 and SF24D models. Yet is not the shutter speed constrained when the flash unit is being used?
Elsewhere, I read that the light meter in the M6 should be ignored. Now this statement of the light meter being ignored makes sense especially when using the TTL mode because the flash adjusts to the distance and aperture.
I'm sticking with this "Most TTL flashes will let you choose a range of apertures - you just need to remain aware of the range of distances that will give you."
It just dawned on me now, sorry, that the m6 classic is different from the m6TTL so the m6 classic would have to be shot in manual mode because the classic does not come with the TTL. Yet both have to be shot under 1/50, so it being TTL or not does not answer what the author in the quote is talking about.