Why is macbeth at first confident that he will survive?

When visits the witches a second time, they say that only when Birnam wood marches towards the castle and only man not born of woman can kill him, it's like a confident boost because these things seem impossible.However, the woods look like they move as an army approaches Macbeth's castle, and Macduff (who he is warned of) was cut from his mother's stomach, not physically born.

Hope this helps!