I would tend to absolutely disagree, in a large number of cases.
First, your analogy is a bit flawed - this player you allude to sounds like much more of a stealthy raider, than a turtler.
But it's a simple fact, some nations in the game start out of the gate with more overt strength. Others are not as immediately capable, and must rely on strength in numbers, research, etc to gain a decisive victory. There is a large gamble in turtling in this scenario. With the victory conditions the way they are, someone could theoretically just sit there while an opponent they can't even see wins the game.
There are many styles of nations in this game, and many styles of players - the very fact that you have trouble discerning their motives, means some of them are definitely doing something right.
