Why are the cold weather underwear called "long johns"? Was the inventor named John?

Long John's foundations lie in the heart of Derbyshire at John Smedley Mills, Matlock. The company have a 225 year heritage and are said to have created the long johns, named after the boxer John L. Sullivan who wore the long johns in his matches. Still available today, the company pride themselves on delivering fine gauge knitwear and their sea island cotton which have they have become renowned for worldwide.

According to Michael Quinion, "John" may be a reference to boxer John L. Sullivan, who wore a similar looking garment in the ring. This explanation, however, is uncertain and the word's origin is ultimately unknown.

https://en.wikipedia.org/wiki/Long_underwear