It's a half-step away from the WCIII DK, made mostly in the name of opening it to other races - and Blizzard have tried, very very hard, to keep it as close as they can to the original. Meanwhile, the Horde's DKs were a completely different sort of monster entirely. It's a completely uncrossable gap between two of the generations - and a vague one not even really acknowledged by the Death Knights themselves between the other two.
I honestly am trying to figure out where it came from, though. As in, the actual term - "Third Generation" - not the fact that the Ebon Blade are a bit different (which they are). I just don't get how it could've gotten popular enough to get mentioned repeatedly in just one page of this thread if it's really just fan-made. I feel like I'm definitely missing a quest - or a comic here. It can't just be that people repeat it again and again, can it?