I think that's harsh mate. The DTV is not an education visa. They want to encourage Muay Thai study combined with tourism.
Many of my Muay Thai Fever applicants are doing there course with flexibility
Our 6 month course gives them 26 classes but can be done whenever they choose. Do a month training, travel a month etc is perfectly acceptable with the DTV
If the gym write a letter and include owners ID card, ltd company certificate and various sports authority licenses, (which is unlikely from a corporation like virgin) then you are still taking a chance as most embassys want soft power activity... but it is possible to get non soft power, it's just a bit of a risk as they may decline
Fairtex is a legit gym, one of the most well known gyms in the world. Maybe this package comes with a lot of training bolted on. Not all muay thai DTV courses are the minimum attendance just to secure the visa. Some are bona-fide courses aimed at actual muay thai enthusiasts
If it's just visa and no training I doubt anyone will take it at that price though.
My gym does 1 class a week for 6 months, 15,000 baht. It is enough to secure DTV but we can do more expensive courses for people who want to train more regularly