Having all that stuff is bad enough. Using it effectively is another thing. The MGs and small arms I can see them using. Vehicles need fuel, maintenace, parts (they can scavenge a lot of that, granted), and skilled operators. Heavy weapons need stocks of ammunition, specialized equipment, and more people with skills to employ them.
I see them selling off anything they can’t put to use. Possibly for pennies on the dollar.
“22,174 Humvee vehicles, nearly 1,000 armored vehicles, 64,363 machine guns, and 42,000 pick-up trucks and SUVs.”
22174 Humvees allows for a lot a scavenged parts. As you stated.
They can probably keep a third or more running. Same with the machine guns, SUVs, etc.
The Afghans are pretty good at keeping old Toyota/Datsun/Nissan pickups trucks running. Some are from the 1980s.