Think its the misnomer that is confusing everyone here.
It should be termed 'constant aperture' instead.
Yes it should.
I think it is termed as constant aperture, but many rephrase or put in place a synonym, that is "fixed" instead of "constant". Theoretically, it isn't incorrect, as your aperture is a fixed maximum aperture of f/2.8, but it isn't a fixed aperture of f/2.8 only.