I can believe they quote those in your list. I believe they think those in the list were or are intelligent human beings. But does that equate to being thankful?
Leftists only use the writings or sayings of the people mentioned to excuse their actions, to justify themselves.
Thankful? No, not really. Being thankful requires the ability to think beyond oneself . . . . the ability to believe that people can do something for someone else without it benefitting themselves . . . . selflessness.