There is some truth in the previous posts, and also some incorrect information. Here is what they are REALLY doing at Texas A&M. Their "secret" is not just one thing, it's a combination of many things.
A. They recruit elite 800 runners. Donavan Brazier, Athing Mu, Sammy Watson, and Brandon Miller were the best 800 high school runners in the country. Devin Dixon, Jazmine Fray, Bailey Goggins, and Dominique Mustin were top 10 in the country in the 800 in high school. Avi'Tal Wilson-Perteete, who just transferred to Texas A&M in January, was the top college 800 runner in the transfer portal. Any college team who can get superstar talent like the 9 runners I just mentioned, is going to have a lot of success.
B. They recruit only 400/800 runners. In other words, they don't recruit 800/1600 runners like most universities do. This allows Texas A&M to have just one type of training, because all of the 800 runners are built the same way, meaning they are all really fast at the 400. (Miller, Mu, and Fray were all part of national championship winning 4x400 teams. Brazier is on the USA 4x400 at the World Indoor Championships.)
C. They train the 800 runners separately from the rest of the runners at the school. At many other schools the 800 runners train with the distance team. Not at Texas A&M. The 800 runners are their own training group.
D. They focus on quality over quantity. Some schools have their 800 runners running 8-10 miles per day. Donavan Brazier has said a number of times that he has never run an hour straight in his entire life. The weekly mileage for 800 runners at Texas A&M is around 30 miles per week. In fact, one of the reasons that Wilson-Perteete transferred there is because she hates distance training, and at Texas A&M she wouldn't have to run much distance.
E. They have a great training system for 800 runners that they have perfected over the years. Coach Mallard has only been at Texas A&M a few years, but his 800 training system is one that Coach Henry, and the previous 800 coaches, have perfected even before Coach Mallard got there. I don't know if it's Coach Mallard or Coach Henry who decides on each day's workouts, but either way it's using a 800 training system that was in place before Coach Mallard got there.