Do wingtip vortices really only form when airplane wheels are separated from the ground?
You are absolutely right. The vortices form whenever the wings produce lift. Even if the lift is not enough to let the aircraft fly.

Of course it is necessary to have a flow across the wing, and with very low speeds (like taxiing) the vortex is likely to be very small.

But,
it seems your textbook is trying to establish a save way to avoid the vortices of previous aircrafts. And since the strength of the vortex is related to the lift which is related to the air-speed and angle-of-attack this rule of thumb is quite practical. Especially since landing is a highly transient process which is very likely to result in a significant reduction of vortex strength.