No they didn't. They could have contributed to GCC. That would have been extremely welcome and it would have helped the whole world.
Think of it this way. With the GPL, you start with the base A and let's say there are three contributors B, C, and D. The result it ABCD.
With BSD licence, B, C, and D each keep their work secret, so the world gets three compilers, AB, AC, and AD.
The result is crappier compilers, which is bad in itself and also creates less motivation for E and F to join in.
Intel chose not to help, and would only help on condition that we don't benefit from their work, but dozens of other companies are helping, so we don't need Intels non-contribution, and we do appreciate the contributions of the other companies.