While nobody has officially send troops, there are a significant amount of professional soldiers from NATO states in Ukraine. Some of them volunteered sure, but I would assume that there are also special forces there, for let’s say payed time off. This however does not change the original point, that the use of 10000+ NK soldiers changes the dynamic of the war.
I assume they believe it makes them go faster? I have seen videos where they drop bodies out of a truck, apparently in an attempt to get lighter and therefore faster. It’s not particularly smart, but other people have just offed themselves after seeing a drone nearby. So there is a spectrum of reactions.