Fact or misinformation?

No, he may be confusing it with surface noise. Early mono pressings were played with 1mil conical stylus while with stereo the stylus used were 0,7mil and played monos fine. You can destroy grooves as @intactaudio said.

A small exception is dirty 78's, where groove width is much greater and a modern stylus will reach the bottom of the groove and not only.

Probably he was refering to that.

Guy was perhaps a dealer looking for sales?

Can  playing a mono LP possibly damage a stereo cartridge?

i always have a fire extinguisher on hand when i listen to mono lp using my art9

"i always have a fire extinguisher on hand when i listen to mono lp using my art9"

 jss49- yes, the mono LP's I play on my ART9 sound amazing. IMO, best "value"

cart that gives you an idea what the hyper carts do.

Dave is entirely correct. There is one instance that might damage a stereo cartridge and that is trying to play 78s with it. The groove size is much larger and a modern cartridge will bottom out in the groove encountering more friction than usual as the point digs into the shellac. 

I do not bother with mono cartridges. A stereo cartridge will play mono records just fine. Imaging is not important as there is no image. Mono records actually sound best off axis.