Field ids are not (necessarily, especially when doing localization) something shown in the UI. The point made by the original commenter is that a field in the error should refer directly to which field has an issue. It does, via an id that happens to be "email". It's still up to the clients to decide how to represent that to the user, but they're given a distinct field rather than needing to infer which field an error code refers to.

(While the comment I replied to can be read differently, I assume we all know that changing actual field names (in APIs) depending on localization is nuts)