Voyager has an omnibus account with the Metropolitan Commercial Bank where Voyager's customers deposit their money. Voyager acts as the money manager of the omnibus account and has absolute control over the money.
Metropolitan is a member of FDIC and is FDIC insured. In the case of the Metropolitan bank failing, the FDIC insurance kicks in to cover any loss of the omnibus account upto 250K. However, Voyager is NOT a member of FDIC and is not FDIC insured. In the case of Voyager failing, the money is gone. FDIC won't be involved as Metropolitan is still fine.
Voyager customers hoping FDIC coming in to cover their loss are going to have a rude awakening.
Edit: A simple analogy is that your parents and you open a shared account with a bank. Your parents lose all the money from the account. FDIC is not going to cover any of the loss since the bank is fine.