As far as I'm aware, it's always been the other way around: Miri adds some features on top of rustc's const-evaluation code. The limitations of the latter are mainly self-imposed, due to the issues of exposing the different runtime models to programs. (E.g., you don't want to create allocations in const code that get deallocated at runtime.) Indeed, since 2019, the full functionality can be exposed with the unstable -Zunleash-the-miri-inside-of-you flag [0].

[0] https://github.com/rust-lang/rust/pull/56123