"I'm wary of online descriptions from manufacturers claiming bags can take huge amounts of equipment when they evidently don't have sufficient space."
Even if the bag can take "huge amounts of equipment" there are two more pressing real world problems:
Problem 1: Can you carry it comfortably for even a short distance?
Preblem 2: How easy is it to get to just the thing(s) you need really fast without having to dig through the other crap or spread it out on the ground?
I have a KATA Revolver backpack. It's a very decent backpack with a clever idea built in: a vertical "lazy susan" turntable that you spin to get a specific lens out. Still most times I go out shooting I use the older Thinktank Photo Shapeshifter and the two primary reasons are fit and comfort when it is fully loaded and utility and organization when I or my assistant are switching bodies, batteries, lenses or speedlites.
I first gave the ShapeShifter a workout while shooting in the streets and souks, food markets, and holy sites in Jerusalem: these are all intensely crowded places and I was out in them from dawn to dusk for nearly a week with three camera bodies and lenses from two different systems, plus my RRS pano gear. Since then I've taken it on multiple industrial, B2B, and editorial shoots across the USA. TTP's motto is or should be "designed by working photographers for working photographers."
I have no doubt your friend is equally as happy with his Kata Rucksack as I am with my TTP ShapeShifter. Everyone is different. In today's market I have no doubt you will fine something that is exactly suited to you .