This is just a difference between named and anonymous types, really. This is very similar to named functions vs lambda functions, both provide the same exact functionality but avoiding superfluous identifiers makes programming more ergonomic.

More specifically sum types in Rust must also be tagged, meaning you need to explicitly construct and deconstruct them. This aspect is along the type-alias vs newtype axis which gets into structural vs nominative typing and the trade-offs therein.

So yes, Rust enums and Typescript union types are both exactly sum-types and the differences are due to the surrounding decisions made about the languages. Rust's sum types are named and tagged, Typescript's are anonymous and untagged, but they're both sum types.