I'd love any evidence of that and I'd wager that that statement is wrong. Blocking the box only accomplishes slowing down the cross traffic with no benefit to the person doing the blocking.
I don't know about Boston, but I think there's no question blocking the box in isolation gets you there faster, although the difference may not be meaningful.
If the exit moves shortly after you blocked, you may have saved the time between when you got to move and when the light you should have stopped for turns green. If it's a long cycle, this could be meaningful, although it may depend on bottlenecks elsewhere in your path.
If the exit remains blocked until after your light cycles, you still saved the time it takes to cross over half the intersection. This probably isn't meaningful.
Of course, if your blocking of boxes actually blocks a traffic move that would unblock your exit, that's not good. And if blocking becomes common, it causes confusion and delay for everyone.
If someone miscalculates and ends up blocking the box, they'll get honked at, but they still get to their destination faster.
If they hit someone who had a green or walk signal, I don't know what happens to them, but it doesn't seem to be enough of a deterrent.