Here's an ellipsoid in build123d using geometric transformation of a sphere:
from build123d import *
mat = Matrix([[2,0,0,0],[0,1.5,0,0],[0,0,0.5,0]])
ellipsoid = Sphere(100).transform_geometry(mat)
If you are using build123d, probably the easiest way is to sketch an ellipse and revolve it.
The definition of an ellipsoid is that it can have three independent radii. A revolved ellipse only has two independent radii. If only two independent radii are needed then yes I agree with you, but I wanted to provide a fully qualified answer.
BTW: I spent a few weekends playing with Microcad (https://microcad.xyz/). It was cool, and had a similar rust feel. I just, for the life of me, couldn't figure out how to do 3d ellipses.