Isn't that just how type aliases work rather than anything to do with structural typing? That example also compiles in Haskell and Go and Kotlin.
Here's how type aliases are usually documented: "Type aliases do not introduce new types. They are equivalent to the corresponding underlying types."
The person above did have a better example of the downside: You usually want something to nominally comply with a specific interface, not structurally. i.e. Just because something happens to have an `encrypt(u8[]): u8[]` method doesn't mean you want to accept it as an argument. (Go interfaces have a similar issue.)
