The US government doesn’t want to install any software backdoors. They gave up trying that in Clinton’s administration. They just want to know whether any backdoors they might already be using are discovered, or are compromised.
If they wanted to find weaknesses in a released implementation, they can do that while circumventing any need for direct access to TOR’s team.

Any useful implementation of TOR on the Windows platform would need to bypass the Windows encryption API altogether, because of _NSAKEY — there’s zero cryptographic trust in Windows’ architecture. It would need to be signed independently of Windows’ architecture. That will require any Windows implementation to have a low-level cryptography interface that differs in structure and method from, say, a *NIX implementation, where the trustedness of the OS’ APIs can be verified, independently, and don’t contain a known backdoor for which no one is practically or legally accountable.

Given that the us government has an electrical engineering group dedicated to manufacturing and certifying backdoor-free hardware for us government use, then it’s also quite likely that they’re not hoping the TOR team is going to stumble across a backdoor system the USG doesn’t already know is in existence.
If the US govt has a hand in the TOR team, it would be to find out whether someone on the TOR team, while developing a software-based surveillance-avoidance system, finds evidence of an existing hardware-based surveillance system.
The USG wouldn’t be looking to find anyone’s surveillance system or place one, through TOR. They’d be looking to find out if one that is already in place had been discovered, and what intel they could use to prevent that information from being spread.