Sheffield's is a good ethical source for rare seed. I have noticed that Sheffield's sometimes relies on Wikipedia information for rare species. Hardiness might be listed as a certain zone but the reality can be a little more or a little less hardy. Often published hardiness ratings are simply guesses from where a tree's native habitat is. Which can be misleading.
For example Osage orange, Maclura pomifera, it's native range was small mostly in Texas, and would suggest it only hardy in zone 7, but it has been planted in all 48 contiguous states and has proven hardy through zone 4.
So view info on websites and catalogs as what is generally thought to be true, but there can be surprises and exceptions. Not everything written in Wikipedia is the result of years of research. Some is just educated guessing.