OK - I'm ignorant and am about to prove it. :-)
Looks like this is just more in the velocity vs. size debate.
When I first heard about this handgun answer to the .223 mouse gun rifle, I thought it was typical of the European approach to police weaponry. Now that Jeff Cooper has passed away, I'd guess that he's rolling in his grave and laughing at such a silly little cartridge.
Forever the Europeans have relied on underpowered guns that have always lacked fast stopping power, regardless of ultimate lethality. They have carried .32 autos, .380 autos, and most now seem to have opted for the 9mm. None of those rounds have a good reputation as an instant stopper.
I suspect that this new mouse gun at 2000 fps will be absolutely deadly if it has sufficiently robust bullets to get the penetration it needs, but I don't see it as a reliable one shot stopper. I've got a .220 Swift and even if the hunting laws allowed it, I'd not be likely to use it on deer or larger game. Yes, modern tough bullets might make the difference, but why take the chance?
On the other hand, as long as we are not talking about self-defense, I'd bet it would be a fun gun/cartridge combo.