The items you become proficient with due to the feat may or may not be shields. An Extreme Shield obviously is a shield while a Gnome battle Cloak is not.

While excluding exotic shields from shields obviously is the intention of the rules, there is no actual rule saying that exotic shields are not a subset of shields, unless the description of the individual item actually says so.

Look at the tower shield. It obviously is a shield and characters only do not become proficient with it, because for some classes it is explicitly excluded from the rest of shields. Exotic shields are not generally excluded.