There's usually a walk/path of some sort, but they never elaborate on the specific science of it. I usually equate it with the idea of wormholes, but...robot science.

The way I see it is this- since Elian's able to go through the spacebridge without losing Echoes, it doesn't count as being outside of the boundaries. (Especially since it fails if he attempts to use it to travel outside). Or, hell, the whole Echo boundary carries in to it, like poking a hole between two sealed containers and letting air travel between both but nowhere else.

I'm fine with it being either way! If it's too much of a headache let's just call it 'nope' and be done with it. I thrive on this kind of thing, myself, but I know it can be confusing.