These numbers matter more here. The Cummins/Allison engine/drive-train in these vehicles are otherwise good for 1 million miles before rebuild on average until they are used as delivery trucks. The constant stopping and starting used as delivery trucks cuts that number down to around 480k miles. So they are selling vehicles that will require engine and transmission rebuilds in less than 80k miles. That's very shady. The engine rebuilds are usually around $10k same as a refurbished engine and the transmission rebuild is around $3k. That does not count the cost to install them.
The consensus seems to be that after 500,000 miles the Cummins engine will either be dead or need to be rebuilt. 400,000 miles is considered high mileage that few achieve.
So on the surface it seems like you might be right that LinuxBender was wrong. But he did sound quite knowledgeable, didn't he? So I still can't tell one way or another. Ah the beauty of the Internet :).
But ive had numerous 90s+ low trim consumer vehicles breaking 300k miles easily and I know some semi-trucks do have 1 million+ miles on the original motor because of such features. And so while I don't have much experience with commercial delivery vehicles, I don't see any big problems with saying they could go a million miles themselves, although due to the nature of delivery driving and driver turnover I would also expect 400-500K miles to be when a rebuilt should be considered unless it was all done by one driver who doesn't drive like mad and they had skilled mechanics maintaining them. And if they didn't care about ruining the engine at some point I would expect 6-700K atleast if it was a well built commercial engine with commercial features.