You're right that this tip will only work when the line is quite short - or it may also work if you have FOTL privileges. In that case, it saves you the time of walking out and around, but it can't be used to jump ahead of guests that you wouldn't otherwise be allowed to jump ahead of.
There is a similar tip that *will* work all the time. If you exit the attraction by the normal ramp to the left, shortly before reaching the gift shop you'll come to an ordinary-looking door on your right. Pretty sure it has a push bar on it. If you go out this door, you'll find yourself in the area for the single-rider line, and you can head back in with no questions asked.
This one doesn't save you as much walking, but it can be used pretty much all the time. Even if you don't want to use single rider, I think from this exit it's not difficult to get back to the regular entrance, so you've still saved a little bit of walking.
Does the trick work for Single Rider?
As a single rider I use that shortcut through child swap all the time to ride MiB over and over without having to go back through the long line. It was a CM who showed it to me and told me to use it. The CM that is standing there where the express pass and regular line merges, usually just laughs and says "You're back again?". He may hold me up for just a minute, but then sends me through either the single riders or sometimes just tells me to join the regular line there.
Just FYI: there is the same kind of shortcut through the child swap area over at Mummy also. A CM showed me that one too and told me as a single rider I could use it, instead of having to go all the way outside and back through the line again. It rounds up over where the Express Pass people board. However, that CM will usually only let you go through four times in a row, before telling you you have to go back outside and use single rider again, before starting over. lol