It's a relatively recent convention though. Isometric means "equal measurement". The measurement along each axis has nothing necissarily to do with angles.
The only difference is if you "stack" tiles vertically they slightly offset in 2d space from ones that are offset 1x+1y.
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.
The only difference is if you "stack" tiles vertically they slightly offset in 2d space from ones that are offset 1x+1y.