This is just more misinformation, actually. Thanksgiving festivals were common in Europe before the colonization of America. See Lammas and Horkey. The settlers just continued their traditions in America. The native Americans had similar traditions, but the idea wasn't anything new to Europeans. Canada's Thanksgiving has moved around a lot over the years, but its current day was chosen to separate it from Remembrance Day. Its timing has nothing to do with Native holidays.

I don't who "they" are to really respond to the rest of your comment. You're kind of painting the Indians with an extremely broad brush. Almost nothing will be true about all the cultures of an entire continent. The Pilgrims primarily interacted with the Wampanoags, but they didn't have a written language and there's certainly no evidence their tribe existed for 25,000 years.

There's a common belief among the Iroquois that it should be considered how actions will affect the seventh generation, but the idea that that's in their constitution is a common myth. The Iroquois Confederacy itself was only formed about 1450. If you read the Great Law of Peace, it bears no resemblance to the US Constitution. Calling it plagiarism is ridiculous. There are not even any significant references to the Iroquois by Congress in the 1780s. This is another modern myth which originated in the last hundred years. The Iroquois constitution wasn't even written for a democracy.

Cultural appropriation is as old as culture. The oldest example I can think of is any pagan holidays that Christianity stole.