Phones typically have 2 operating systems: one to handle telephonic functions like managing tower connections and separate operating system for user applications. The security implication here is that the telephonic operating system has access to the same system resources as the application operating system at the same time, right? The problem is your data is always available in plaintext to the telephonic OS because it has to be stored somewhere before it gets encrypted, so assuming graphene OS doesn't handle both functions its not a perfectly closed loop system.