That's a fairly common move.
In Star Trek IV, you didn't see people walking up to Spock in San Francisco and shouting "Live Long And Prosper!"
well, i see what you're getting at, but that analogy doesn't hold.
it's not that there was a movie once where they made up a term, 'zombies,' and the creatures themselves, and then this show came along and in some way continued that story or took place in that world.
irrespective of any movies, books, etc., zombies are real. in a sense, anyway. they're in haiti. the western world has known about them for what, 100 years or more?
so this is all a very minor thing (but then, what good intertube discussion isn't?), but yes, when i watch this zombie show, i'm briefly taken out of the reality of the show when characters, seeing the dead walk, say 'we call 'em--walkers.' no you don't. you call them zombies. that's the word!