The grandparent was probably referring to the fact that Rust allows mutation and side effects, which Scala also does. "Functional" is not a very meaningful term

The grandparent was probably referring to the fact that Rust allows mutation and side effects, which Scala also does. "Functional" is not a very meaningful term, but Rust has about equivalent support for higher kinded polymorphism as SML or OCaml (in that there is no first class support but another language feature can be used to implement it), which are certainly functional languages. And of course, many lisps are considered functional languages despite not having types at all.