The previous comments about PCM are due to the modulation and not the failsafe. As guessed from its title, PCM is a code. If interference occurs the rx knows because it does not fit in with the code. The model therefore tends not to glitch. This is nothing to do with failsafe. Failsafe will kick in after a specified period of not receiving the correct code and is another, entirely separate, benefit.

I would be interested to know how you can get the same glitch avoidance on PPM. How can the Rx tell what is interference and what is a command from the TX?

Failsafes can be fitted to PPM receivers but you still don't get the added 'glitch' protection.