That's speculations, Nothing has been stated that they really are training ammunition besides using training ammunition is useless if they don't perform the same manner roughly.
While training cartridges exist it is odd to see that we only saw it being used once in that video and suddenly then never.. but that may have never been the case either. For all we know it could have been proper ammunition.
This is specifically rubber training munitions for firing in indoor CQB practice areas with troops moving through, where this will not affect the training experience, but where safety takes precedence. This is not training ammunition for firing at targets at range. A rough analogy being a much more potent solid paintball but probably still potentially lethal in a freak accident.
We do know of the specific rubber sim munitions used by the PLA for indoor CQB training, which given the context of the clip would make be the most logical explanation. You would not fire live rounds in an enclosed space with other troops moving around. Penetration, ricochet and spalling would be very dangerous.
Rubber munitions will not be able to engage rifling sufficiently to stabilise, let alone the density and other characteristics of a rubber projectile that will likely not allow it to develop a high enough spin and angular momentum to stabilise.
