Why do you say that.
Ignorance on Covid-19 that's how.
How sure are you that everyone that would be at the Thanksgiving celebration were also previously infected?
He doesn't know, nor how many they would come into contact with outside such a celebration.
He just plays an epidemiologist. No training, no data, no expertise; lots of assumptions. He is sure that everyone in the family is immune from Covid-19 and can't spread it.
This is what is known as
herd mentality; a lack of understanding the subject but regurgitating it often.