I would say credentials do indeed matter, although yes they are overshadowed quite a bit by professional networks.

Outside of simply procuring a job, they matter even more. As an example, the company I work at offers professional education services on CUDA and OpenCL (with other HPC architectures in the works). One of the guys who teaches it did his masters in EE at Stanford. I don't think the impact of that can be overstated when selling the service to companies.