Like copying a good book, copying a successful strategy is labor intensive, takes a lot of time to get right, and is rather large and involved. (This is true of copying a strategy; copying a particular set of trades is trivial if you know the trades).

Being a good prime broker is also a lot of work, and is an elaborate and involved process.

Now for the firm to take on two jobs, and then keep it secret... I wouldn't worry about it. It's like worrying your accountant is going to take your successful startup idea and run with it because they know how much money you make.