Wikipedia definitions are just the opinions of whoever wrote the entry. A muscle car is whatever you think it is. I graduated HS in 1971 and any two-door sedan with a high performance or tricked-out engine was considered a muscle car in those circles. The term "muscle" as related to cars of that day and age was pretty non-specific in terms of body style, but always related to performance, mainly drag race. Witness FoMoCo's parts and information catalog from 1969/1970 which was titled "Muscle Parts" and is quite an interesting read. Check out the price list in the back!
https://www.mustangtek.com/Library2/MuscleParts.html