squeak,
I researched further and found potatoes were grown in Europe beginning in the late 1500s (they were native to Peru). So by the time the first Thanksgiving was celebrated they were common in many parts of Europe, and a staple in Ireland. In fact, Irish immigrants brought potatoes to North America in the 1700s.
So you are correct in that potatoes are not native to Europe, but they were actually grown in Europe before the first Thanksgiving. I still think you meant to write “sweet potatoes” and not “potatoes” on that list of foods that were new to the Pilgrims.
I know that the Irish potato famine happened 200 years after the first Thanksgiving. But I mentioned it because that’s what made me think that in order for potatoes to have become such an important staple in Ireland (so much so that a blight in this one crop caused massive starvation and emigration) it’s likely that potatoes had been grown there for some time by then.