It was not just additional fill-in 500 Hz inductors, but also providing one at all in the first place - before that, they were use somewhat more sparingly.
The major difference of PZB90 is the so-called "restrictive mode": Looking at the speeds normally applicable for passenger trains for example, a 1000 Hz activation normally enforces a braking curve down to a speed of 85 km/h. Now if with PZB90 the train comes to a stop (or just crawls along very slowly) while still within the 1000 Hz supervision, this speed limit decreases to 45 km/h. The same happens after passing a 500 Hz inductor - the normal supervised braking curve is 65 down to 45 km/h, but after a stop this decreases to 25 km/h.
Operationally it's of course not the ideal solution, as this causes some slowdowns because of trains having to run at at restricted speeds a little longer even after the signal clears, but on the flip side for a legacy train protection system it's a quite effective solution for ensuring that after a stop (with the overlap having potentially timed out) trains cannot approach a signal potentially showing danger too fast.