There is a real glitch, but it had nothing to do with that truck. I outlined it in a nearby comment. The trick is actually abusing a trainer battle script defect related to how opening dialogue boxes were stored in memory. Essentially manipulating the value to select any pokemon we want based on special values (you could use the glitch to get any pokemon, but mew is obviously the highest value target)

It's not the truck, but Mew is real. It's another swimming along an edge thing but in a different spot than missingno