I haven't touched the wiki article in ages.
I checked, and actually the whole Double-slit method article was written on the 8th of June, then rewritten on the 11th (all by Nagidal, kudos to him), so pretty recently. He also added a link to the youtube video you posted about. The tutorial for the method was probably on the aquifer page before that, because I remember reading some older, more tedious version (the same one that was on the article between the 8th and 11th, I think).
Aquifers can be something like 13 levels deep, I've never counted precisely. However the sand/loam layers are almost never more than 3 layers deep. Once you get into rock, it's easy, because rock can be smoothed. Using twin-slit method, smoothing is SOOOO much faster than digging out and building constructed walls. Also rock layers are rather likely to contain large deposits of ore or other minerals which aren't aquifer bearing.
I guess it's improbable, but possible to be really unlucky and have eg. a 3-layer aquifer, 1 layer of damp non-aquifer, then maybe more aquifers repeating like that, if it's possible, and if you can get to something like 15 z belowground, then you could have really bad luck and have an underground lake right underneath, and so forth...
Can you cast pistons? I realize that you don't need a piston if you have access to the lava necessary for casting, but just wondering if cast obsidian deconstructs.
Cast pistons should work (=stay intact when caved in), since cast obsidian counts as natural rock. See eg. the magma piston with the integrated obsidian caster to make it reusable.