No it isn't! It's supposed to be a new game. Warlords was an upgrade from Civ4. Civ5 is a completely new game, built from stratch. Whether you like Civ5 or not, you should be able to recognise this.
Yes it is!
Seriously then, as there's small chance that I was using a wrong adverb (I don't think so but then again English isn't my first language) let me explain. We have this game called Civilization. This far there has been five iterations of the game. Each new iteration is, or at least it shoud be, an improvement (I was using the word upgrade but I admit it may be a bit confusing when talking about computer programs) from the previous iteration; if it isn't there's no gain for the customer because they could keep the previous better iteration for free.
Warlords and BTS were upgrades to (not from) Civ4 and as of now the fourth iteration of Civilization is Civ4 + patches + expansions. Fifth iteration is Civ5 and what little patches have come to this day. From the customer's point of view only the quality of iterations matter - the fact that Civ5 has no expansions yet doesn't make me enjoy it more, it's irrelevant to game's current quality.
And yes, Civ5 is probably a new game built from the scratch but more importantly it is a Civilization game. Again the specifics of the code are irrelevant when reviewing the game. Whether it's new code or 99% recycled from Civ4 has nothing to do with the matter.
Did I make myself clear this time?