Maybe he could have saved a few thousand with different subs (maybe) but your comments are nonsensical. I see tons of janky builds people post on Reddit and this is not one of them. You should spend your commenting time over there.
My calculus is to evaluate time, cost, and quality. If someone can match or beat me on 2/3 of those dimensions then I'll hire them.
This is the danger of "cheap projects" - they can grow legs before you know it, and you end up paying half the cost of increasing the square footage of the house (or the garage).
You get a finished and conditioned room, and it most likely added value to his house.
You can buy all the Quickcrete you want (not sure the last time you priced it), but it won’t make you good at pouring a level pad. That alone puts me off trying to do it some days.
An 8x10 6” slab has 1.5 cubic yards of concrete. Making concrete takes more than a bag of Portland cement, you also need gravel, sand, a mixer, a wood frame, a source of water, shovels, floats, et cetera. If you want rebar, add in steel and rebar tools and wire.
You could frame up the wood yourself and have Cemstone deliver the concrete but you still need to spread it out, float it, and finish it.
Instead of buying or renting all of that, I would sub it out. I subcontract out light pole bases and concrete pads for transformers, EV chargers, and other equipment all of the time.