if they didn't find it in his lungs, first of all, it'd have to have gone done his esophagus, not his windpipe. it goes down a lot easier in the esophagus.
second, if he swallowed it just before he died, the acids probably only got a chance to start eating away at the outer layer of the tape, not destroying the inside or film.
eventually, the acids may have eaten away at the whole thing, but it probably would have taken days, so it depends when they found him, how long he'd been dead.
and it's a whole other thing entirely if it didn't actually drop into his stomach. then it was perfectly safe and away from the acids.