I checked wikipedia expecting to be able to close the thread, but it may not be so clear cut. It is actually interesting to read the history behind the term:
https://en.wikipedia.org/wiki/Supercar
Perhaps if the individual who is classifying his M3 as a "supercar" is old enough to have been raised in the 60s, I could see where he is coming from. From an old school mindset, there is some argument. If not, then yeah, he's more than likely just blinded by ego.
In any case, by the modern definition of the word, the M3 isn't really even in the ballpark.