Right, but that’s something that will be observed in the making of the fonts, and which the format provides ways to address, i.e. the interpolated corners can be replaced by delta sets. What font makers* and users won’t see is extrapolation beyond those corners, i.e. in the area over which the format provides neither control nor access. [If the format had been designed to enable extrapolation beyond axis extremes and outside the range of the design space, it would then have been necessary to also allow delta sets to be positioned out there also.]
*In fact, font tools may provide extrapolation for font makers in the master design space, but this could only be written to the variable font as a delta set within the variations design space, i.e. the extrapolation along an axis would constitute a new axis extreme instance in the font, increasing the range of the font’s variations design space beyond that of the master design space. All plausible. All quite difficult to explain.