If they're comparing Recon expansion to The Frozen Throne expansion, then what they're saying is that Recon is not going to run on a new game engine, but use the Halo 3 engine, just as The Frozen Throne re-used the same Warcraft engine.
But remember, The Frozen Throne was a VERY large campaign, so comparing it to Knights of the Nine or Shivering Isles probably doesn't reflect how large a game it will really be.
In other words, saying it's not a "new game" is splitting hairs... it will probably be just as large.