Webster is wrong. A mainframe is not a generic high performance computer (that would be HPC). A mainframe is a very specific high performance computer.
I repeat: I understand that mainframe has a specific meaning to many people, especially those who work on traditional mainframes, but I would rather you and the other user to email both Google and Merriam-Webster about their wrong definitions, and not bother me about it. I will correct my usage once they have updated the definition to your standards.