So that guy with the leaking shower wall

am I understanding him right do you think? Tile - Block - furring - gypsum

Not all that sure, he could be referring to cement backer board behind the tile and calling that stone. Otherwise, the water is running through a whole bunch of stuff to get to the drywall.

if there's cement backerboard it still needs a barrier between that and the wall studs

if they didn't want to use a solution like redgard

if it's cinder block then it still required a barrier on at least one side

I agree, but have doubts that the level of water damage they are seeing is caused by water seeping through the tile. Suspect something else is happening, but won't know until the wall is opened up.

imagine that it's a shower

on a concrete pad

with concrete board

and studs into that pad

showering would absolutely soak the pad

and the wall

the studs are probably quite wet

To give an example, the family had a water leak in a 50 year old shower, had been used for decades by my grandmother without problems until she passed away. When another family member used it they redirected the shower head to the valves/handles on the side to wash their hair and water went behind the flooded the ceiling below. Don't know that until you see the leak in progress.

yeah, this guy implies that it's the wall cavity opposite all the pipes

Yup, which is why I didn't say that's the problem, gotta open it up to find the problem.

I'd say most all the water goes down the drain, hardly any is going through the grout, there's some and it will cause mold, but he's seeing a lot and soaking the drywall. Could be a bad drain connection, who knows.