Just wanted to say to the anon that posted this a few days ago, the Pokedex book written by Tajiri, Sugimori and other members of the staff way back in the 90s supports this theory. It says
>In general, Pokemon evolve through growth, but recent studies have concluded that Machamp and Golem have not evolved by growing. It also mentions that no one has witnessed their evolutionary process yet.
So trade evolutions are being alluded to with these two specifically.