It absolutely is an assumption. You are assuming that...It's not an assumption.
And as I said, if the assumption is correct, you're good to go. I didn't say it was a poor assumption to make.The glove box door is there with the information until you know otherwise.
Yes, it is likely that the truck left the assembly line with that information attached, but the youngest of these trucks are 20 years old now, and some of them have had rough lives.
I don't know where specific VIN decoders get their information, but if what you're saying is accurate, then there's no reason to question the validity of the information they provide.Where do vin decoders get the information. Manufacturer specific and DOT regulatory. There is no magic.