Yes, the Ton/Toff requirements mean that you can theoretically supply a 500 kHz signal on the step line and expect the driver’s outputs to respond appropriately, but this is a case where theoretical maximum speed is so far from reality that it doesn’t really even make sense to consider it a theoretical speed. You will not get these stepper motors to turn at 150,000 RPM! Coil inductance and rotor inertia limit the stepper motor’s ability to respond to the rotating magnetic field; if you step the field too quickly, the rotor won’t step (it might do weird things like vibrate in place, rotate in reverse, or seemingly move in a random sequeunce).
The maximum step rate of your stepper motors will depend on things like your supply voltage, the current limit you have set, and the load on your motor. Also, achieving the highest step rates generally requires you to accelerate the motor by gradually increasing frequency of the signal on the step pin. If you want a ballpark figure for maximum achievable step rates, you should look at the pull-out torque graphs on the stepper motor product pages. Given the highest pps (pulses per second) datapoints on these graphs, I’d say you shouldn’t expect more than 10 or 20 revolutions per second. You will not get 100 RPS. I suggest you try it for yourself, starting with low speeds and gradually ramping the speed up until the stepper motor stops rotating.
The signal on the step pin is pretty simple. Just drive the pin high for at least a microsecond every time you want the stepper motor to advance by one step, and make sure to drive it low for at least one microsecond before you drive it high again. For example, if you want to make the stepper motor turn at 60 RPM (1 revolution per second), you could put the driver in full-step mode and alternate between driving the step pin high for 2.5 ms and low for 2.5 ms. This will cause it to take one step every 5 ms, or 200 steps every 200*5ms=1s. The simplest input you can provide the step pin is probably a 50% duty cycle PWM, and the PWM frequency will control the step rate.