Not entirely, those parts can be made from higher strength AL alloys to reduce the metal volume and therefor the mass of the part(s). They can also be made from other metals like Magnesium and Titanium. There are also high strength copper based and nickel based alloys that could possibly be used.
"Plastic" is so generic these days. Could mean anything from the now roughly 100 years old brittle Bakelite to the latest carbon-fiber filled space age polymer.
It comes down to how informed or how much research the design team puts into the materials for the design of the scope parts.