The Marines had more trucks than they could use, and weren't breaking them fast enough to need all the spares they had.
Then they started getting ECV/REV trucks, and weren't breaking them fast enough to dip into the stockpile of older trucks.
So yes, many of these trucks really don't have any miles on them.
However, many of them went through rebuild (either full rebuild or IROAN (Inspect, Repair Only As Necessary)) and have rebuilt engines/transmissions in them.
For example, my truck is a 183XXX serial number, went through rebuild in 2010, and has an engine with a casting number on the block indicating it was manufactured originally in 2005. So no way to verify how many miles were on some of the parts, but the rebuild shows at 26 miles and at pickup it had 32.