As an employee he can be ordered to testify.
As an ex-employee he can plead the 5th all day long.
As an employee he can continue the coup and cover up things.
This secrecy is BS.
The government might be able to compel employees to testify, and government lawyers can advise them what to say, but the government can’t stop them from pleading the 5th. That would be as clear a violation of the 5th amendment as could be imagined.
Now, if he was granted immunity (with the proper scope), then he would not be able to plead the 5th.