It's not exactly a secret, it's even on the official MBTA map.

> Maps never recommends using the concourse or anything like that

Probably because it almost never makes sense to switch from the orange line to the green line if you're not coming from the north (and then you'd switch at North Station or Haymarket).

When it's busy, it's faster to take the concourse to get from the Red Line to the Orange Line, but you need to be in the right carriage and know exactly where you're going.