「It isn't as if ～」は「～というわけじゃあるまいし」という意味の慣用表現です。It isn't を省略して「As if ～」という形でもよく使われます。
It isn't as if she is a novice.
彼女は新米でもあるまいし。
It isn't as if you were poor.
君は金に困っているわけではあるまいし。
It isn't as if she didn't know anything about cooking.
彼女ときたら、料理について何も知らないわけじゃあるまいし。
It isn't as if I hadn't warned him.
彼に警告しなかったわけじゃないのに。
As if he were really somebody!
彼は何様でもないくせに。
As if you don't know that!
知ってるくせに。
Why doesn't he come? It isn't as if he had no time.
彼はなぜ来ないんだろう。時間がない訳じゃあるまいし。
As if you were a doctor!
医者でもないくせに。