I agree with the vendor. To me anything smaller than a weaned rat is just a "milk bag". Their bones, organs and muscles aren't fully formed and they are filled with mothers milk which isn't good for a snake to begin with. I always feed my babies hopper mice up to large adult mice. Then I transition them over to weaned rats and then onward up to large rats. If they get large enough to eat larger than a large rat, I move them to rabbits as jumbo rats tend to be fatty old rats which again aren't good. This mostly applies to boas though as they don't digest fat well. Pythons can digest fat better but still I wouldn't want to subject them to any more fat than was needed and if I could, I would avoid a lot of it. Some fat is good and needed though.
And before someone chimes in about them eating baby rodents and stuff in the wild, true but they also eat lots of other stuff which we don't provide. A pure diet of baby rodents for a year straight plus not exercising and just sitting in a 3x2 or 4x2 cage just compounds that problem.