In which case, mutatis mutandis for the halfing in the corridor. Eg the halfling is able to duck down low below the flames.Gygax's example with the rock face assumes that such a determination has not already been made. If it had already been made, he would have had to come up with a different example of how the bound PC made that save.
That's the point of invisibility - it plays rather like it works in Tolkien, for instance (when the ring-bearer turns invisible, people get confused and wonder where he has gone - they don't just start guessing what square he is in).So you have an issue with a breath weapon having the one additional property of anti-invisibility, but you don't have a problem with invisibility having the myriad of additional properties of, breath weapon immunity, fireball immunity, sleep immunity, cone of cold immunity, ice storm immunity, meteor swarm immunity, flame strike immunity, gaze attack immunity, lightning bolt immunity, web immunity, cloudkill immunity, and much, much more. Seems odd to me.
It's not inherently better or worse to run invisibility that way, or the 4e/5e way (as least in my view). But they're not the same way.