I think whoever wrote it just used the wrong word because yes inertia is dependent on mass, not momentum

Okay I actually decided to check and I think it just a poorly written explanation of rotational inertia

A rotating spheroid will oblate when its centrifugal force generates enough ~~inertia ~~ angular momentum to slightly flatten it out into a more irregular, elliptical shape.

If the shape also changes, this would mean distribution of that mass relative to the axis of rotation would also have changed, which if I’m reading this right, affects rotational inertia.