I am not assuming anything. Even if the chainstays are parallel as long as possible the clearance required to fit a 700x40 tire and go to market require there to be enough room for a 650bx47 tire. No production frame is shipping with less than 5mm between each side of the tire on a 700x40 tire combo. The vast majority of "parallel" chainstays are not actually such either, they almost always flare directly behind where a 700x40 tire would fit, just extremely shallow flare. Enough to get an additional 1-2mm at 650b diameter.

I'm not speculating here, I see several different gravel bikes every week and have mocked up dozens of designs in bikeCAD. The math the would make a frame fit 700x40 and not 650bx47 does not work except in the more uncommon edge cases.