The USTA is not like the DoD, it is more like the USOC.
The USOC hired an European woman as the women's soccer coach, and after all the success, she has now moved on to better opportunities. They also hired former Soviet-era gymnastics coaches on contract to boost their players.
Brad Gilbert, one of the coaches listed in the letter, was appointed as Murray's coach by the UK LTA. Many US tennis coaches are teaching abroad by contract with government sports bodies. Michael Chang approached the Chinese TA to coach juniors (and was snubbed by them).
This kind of thing is routine in high-level sports.