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Does this mean I can run any servo over it's Rated Speed or are "spindle motors" a different motor type that allows it?
ie. synchronous for positioning, asynchronous for spindle?

Both types of motor can be run above their rated speed however the difference is how long for.? An Axis servo can typically be run 100-200% above rated speed and torque but only for a few seconds. A spindle Servo motor is nearly always running above its rated speed but with a loss in torque.

How does it work.?
Now, I'm no Motor expert so don't quote me, but I think it comes down to the Motor type and how it's constructed.

An axis servo motor type is (synchronous) and uses permanent magnets to drive the Rotor, this is why it can give constant torque through its speed range.

The Spindle Servo type is (Asynchronous) which has no permanent magnets and relies on the rotating field to produce a magnetic field. This means it can run at higher than rated speeds constantly but at the cost of lower torque than at rated speeds.

There are other differences in how they work regards the drive. An Axis servo is typically run using POSITION mode whereas a Servo Spindle is run in SPEED Mode. (There is also TORQUE mode but POSITION and SPEED are the most commonly used.)

This is why you can rotate the shaft of the Servo Spindle motor when powered up but you cannot spin the shaft of an Axis servo motor. The Axis servo drive is constantly monitoring the rotor position and applying corrections to keep it POSITION, if you turn it by hand too far it will fault the drive.
Now, if you ran the Axis servo in Torque mode then this is similar to POSITION but now the drive monitors the TORQUE and tries to maintain it and if the torque falls below a set range it will fault.

SPEED mode you can probably guess by now.! The rotor position isn't monitored at all and is free to spin without any faults in the drive but the Speed is monitored and the drive will try to maintain this speed and again if it falls below a preset threshold the drive will go into a fault.

Now Servo spindles can position the rotor shaft for things like tool changing, they do this by switching into POSITION mode, and using the encoder they can rotate the shaft to an angular position.

So in Recap, Axis Servo can run above rated speed/torque for limited time periods without loss of torque. Spindle Servo, mostly, constantly run above rated but with some loss in torque as speed increases. This due to motor construction, (synchronous) and (Asynchronous)

Hope this helps.