I take extrapolation to imply something beyond the defined range of the axes, and hence I think it makes more sense to talk about the corner space as interpolation between the axes than as extrapolation. The axes define the range, and the corner space between them is implied. The actual corner can be either interpolated or can be an additional delta set, but its position is known from the range of the axes and we’re not going beyond that position any more than we’re going beyond the range of the axes orthogonally. Everything we’re doing is within the variations design space defined by those axes.
I understand that there is an argument for calling movements to the sides of axes extrapolation, but I worry that this terminology will be confusing to font makers, who already have an understanding of extrapolation from font tools as moving beyond axis extremes. If we start saying that OT Font Variations uses extrapolation, we risk people thinking that it supports that kind of extrapolation outside the design space. It doesn’t, and if it were to I can imagine a lot of very unhappy type designers. Faux bold and faux italic are bad enough. I don’t think anyone wants to see a faux ultra black of their nice text face.