There isn’t a Streamer client. There is the Praetorian, internal testing version of the game. There was a Praetorian recruitment session for FoA testing some time ago, as there is with most major content production:
Praets have been given permission to stream certain things in the past, now including the Berzerker mastery.
That which any Praetorian is streaming can safely be considered WIP content and is not indicative of the final product, nor do Praetorians have permission to stream everything currently in testing.
And any non-Praetorian somehow streaming Berzerker content should definitely get a hard looking-at.