Does the presence of a file indicate a "supported" MCU in the IDE?

Is this a question for the Arduino Team or just a community question in general?

The answer to the question "if the MCU is supported by Arduino" is a very large NO as there are no boards with that.

If the question is "could the Arduino AVR core be ported to this MCU" the answer is yes. "Extensive work" depends on your experience of AVR and the Arduino/Wiring source code.

Cheers!