Socket 7 is a physical standard for the socket. That is the number of pins, spacing, how deep etc. It does not dictate what you plug into it beyond those parameters.
In other words, just becuase the socket will take the chip doesn't mean that the board the socket is attached to will accept it.
The mdeia gx chipset is a multimedia chipset with functionality split between the various components. You can run a faster media gx chip, but it has to be a media gx chip.
My guess is that Cyrix (as it was then) made that part of the set a Socket 7 because there were already sockets, design tools, and chip carrier manufacturing capabilities available.
I too have one of these units, and if my memory is flawed in this (it was a few years ago when this was a new product)I apologize and would be the first to rejoice at firing up a K6-3 450!