no it's not, it's a cut voiceline that they left in. reason it plays when you go back is because it presumably was meant to play AFTER the voiceline you're blabbering on about but moved the trigger or sm. idk why why it plays but it does
I It has some possibilities but I would say this probably refers to the fact that Chell just jumped out of a quite high broken staircase, he realized this could happen on a bottomless pit and the triggers were left there right after the fact, but it won't activate for some reason until you hear the following one!
His point was that he's been using the rail to move around, and since he's now out of his rail, he has to rely on Chell for that. Not much deeper than that, as far as I know!