FYI, the whole "silicone are bad for running rubber tires later" thing is specifically in reference to tracks that have been "rubbered in" by consistent use of rubber tires. Most of the time this is for wood tracks, but also applies to painted plastic, and even raw plastic. If you ONLY run rubber tires, depending on the softness of the tires and the nature of the track surface, eventually a thin layer of rubber will be laid down on the track, especially in curves and exits from curves, but can be all over. Traction increases considerably when the track has a layer of rubber, but mainly for rubber tires. Silicone tires will "clean" the surface, including the laid down layer of rubber. As such, the traction will DECREASE for rubber tires as the rubber on the track is removed by the silicone tires.
Remember, though, this is primarily about magless racing as well, where traction makes a LOT more difference. Obviously, wood tracks would be magless racing.
Also, this is only important for tracks that are ALREADY rubbered in, or the owner WANTS it to get rubbered in. If the owner doesn't care about a layer of rubber, or keeps the track especially clean, or wants it to be clean, then silicones are fine, since they'll help clean the track. In this situation, what you found is also the case, where a clean track is better for the rubber tires than a dirty/dusty one. But those same cars with their stock rubber tires will run even better (i.e. with more traction) if the track were to get rubbered in.
My analog club has more or less banned silicones for series racing because there are 2 wood tracks (soon to be 3, no, not me ;-) and the owners don't want the rubber to be taken up by silicone tires. We either run stock tires, soft rubber replacements (Slot.it P6 are popular), or urethane. Urethane don't lay down rubber, but they also don't take rubber off a track. I don't mind so much, but it does mean that it's always a good idea to clean your tires before your car is put into the pits for the night's racing. We're not allowed to clean them during or between the heats.
Anyway, there you go.