i would say calling this projection isometric is 'a relatively recent error'

in an isometric projection all the edges of an axis-aligned cube are the same length. in the axonometric projection discussed here, which is dimetric rather than isometric, the vertical edges are of length 1 while the horizontal edges are of length √(1 + ½²) ≈ 1.118

is the fact that this projection is not isometric the reason zaxxon was called zaxxon and not zisom?

The horizontal edges are clearly measurable. The vertical (z) axis you are making a requirement that it has to show directly on top of a tile 1X and 1Y offset. That's on you.

?

i have no idea what you intended to say here