No, Mettle is not a bank at this stage.
Mettle provides an E-money account with a sort code, account number and card. This means the deposits are not FSCS protected. However, Mettle is backed by NatWest, and all deposits will be protected via a ring fenced account set up to cover the total balance held in Mettle at any one point.
Should there ever be a problem with your account and balance, you would be fully reimbursed.