I fried my spindle motor, and would like to understand why

[m_c]

The motor states 300hz max.
This kind of thing happens when you mix and match.
Should have got a matching vfd from isel.

May have broke both.

But 300Hz would only give 18'000rpm (ignoring rotor slip), not the indicated 23'000rpm in the torque curve.
Going by the same torque curve, I'm guessing what they mean by the cut-off frequency is the frequency at which the spindle enters constant power output, in which case Base Frequency should be set to 300Hz, with Max frequency being 400Hz.

The only way the spindle could be damaged that quickly, is if the VFD was driving too much current through it.
To do that would take the power/current being set far too high (would need to be using a well oversized VFD), the initial test speed being quite slow (or the spindle being locked/stalled), and the base frequency being set too low (even if the power was set too high, the back emf from an undersized motor would automatically limit current, unless the base current was set too low, which would mean the VFD would output a higher voltage than needed and drive excess current through the spindle).

There is also the possibility DC injection could cause the same result, but again would involve wrong power settings and an oversized VFD.