The frequency is not much of an issue - the spindle is typically a 2-pole motor, which basically means each complete 3-phase cycle1 = 1 revolution. So at 300Hz (300 cycles/sec) = 300 revs/sec. 60 secs in a minute = 18000 revs/min. If you drive at 400Hz, 24000 revs/min. Yes, you can drive the motor at 18000 rpm or 300Hz - that works.

With the original spindle - the VFD is rated correctly for the 800W spindle (or thereabouts).

The problem is the new spindle is 1500W, which is likely to damage the VFD. That you can't spin the spindle as quickly as its designed is another issue, but wouldn't damage either spindle or VFD. Why would the supplier provide that VFD?, you'd have to ask them, but usual suspect would be cost.