Simple question.
As far as I know, all races exept:
Undead.
Orc.
Bloodelf.
Human.
Dwarf.
Were able to be turned into Death Knights, by the Lich King.
(note: every race could be Death Knight, but only the members of the above races were turned into Death Knights afaik)
How can Taurens and Draenei have anything lore-related to the deathknight?
Or even worse:
Gnomes.
Only reason (that I can imagine) why Blizz would allow all races to be death knight (atleast durning beta), was to allow more feedback, to get rid of the bugs.
Lore-wise..No, can't possibly be
EDIT:
Found this interesting article:
Why all race Death Knights make sense from a lore standpoint - WOW Insider
It could very well be true, although I need to be convinced.