I didn't hear it the first time either, so don't feel bad. Try to start the playback of the recording link I provided above by hovering your mouse over the timeline line at 4:23. Play it until 4:25 (by 4:25 it's done, so there's no point in continuing to listen).
On my computer, the left hand rootless voicing occurs between the 4:24 and the 4:25 mark. Sometimes I wish YouTube had 10ths of seconds display like the film industry (which has down to 1000ths of a second right?
) , so sorry I can't point you to it any more precisely. On my computer it happens just before the clock marker turns "4:25").
You may need to turn up the volume and try to "tune-out" the right hand for a sec.
Even if this voicing was not there, like you said, it is a common practice to go from Ab7 to G7 in a musical situation like this. The way that particular voicing feels to the hand of a pianist makes sense in this example. A second later, even softer, Red plays a G7(13) during the following G7 measure. This looks like F-B-E on the piano which is very widely used and practiced among jazz pianists. To proceed this with a chromatically higher Ab voicing (Gb-C-F) was probably so ingrained in Red's mind and playing style, that he probably did this as a left hand 'reflex', more than thinking "Oh, let's do a tri-tone sub here". I don't want to put words in his mouth (or notes on his piano - ha), but this is probably what's going on in this brief moment of his improv.
Let me know if you can hear that voicing. Maybe in the spirit of upcoming Halloween, we should nickname it "Ghost voicing".