Not entirely. It makes it so that, to achieve a "full" collision, you have to ensure that the sets of data collide both in SHA hash and in length, helping to prevent attacks that rely on appending/prepending/removing data (for example, "length extension attacks" involve manipulation of the hash by appending data).
TL;DR: It is harder to find a collision SHA(B) for SHA(A) if you add the additional constraint that the length of B must match the length of A.
Not entirely. It makes it so that, to achieve a "full" collision, you have to ensure that the sets of data collide both in SHA hash and in length, helping to prevent attacks that rely on appending/prepending/removing data (for example, "length extension attacks" involve manipulation of the hash by appending data).
TL;DR: It is harder to find a collision SHA(B) for SHA(A) if you add the additional constraint that the length of B must match the length of A.