It is. A titan's weak point is its nape. If you don't cut a deep enough cut it'll just keep regenerating. We have found other means to fight them but this is the most efficient way still.

[Even if by far the riskiest. Thunder Spears are a different story that he'd get into some other time, especially since he only knows the theory behind them and hasn't personally used them.]

I haven't actually killed titans solo, you know. It's a group effort usually.