This presumption is entirely correct. FCSC protection for banks in particular exists to prevent bank runs (too many customers attempting to withdraw their money at the same time) which causes the bank itself liquidity problems that can lead to the bank failure. If a bank fails, then customers lose money. Therefore, preventing them is the primary form of protection and this is what deposit insurance does (stops perverse incentives of depositors lining up outside the bank all trying to withdraw their money) because you know that with FCSC protection you don’t need to worry because the governments guarantees that even if the bank were to fail, you wouldn’t lose any money.
Essentially if Revolut or the company behind their segregated accounts failed, your money would not be lost. It’s protected by a ring fence so that after all the necessary admin/litigation has been processed, your money will be returned to you and not creditors. This is the case with Glint Pay (another app that i use) and Monzo before it became a licensed bank. 
Maybe @simonb could confirm this.