This terminology is regional preference. I grew up in the South, and the “noon meal” IS “dinner”, and we eat “Supper” in the evening.
Somehow, “Thanksgiving LUNCH” just doesn’t sound right.
So, I looked it up:
dinner
n. The chief meal of the day, eaten in the evening or at midday.
n. A banquet or formal meal in honor of a person or event.
n.The food prepared for either of these meals.
In agrarian America - until say 1960 - lunch was the main meal and dinner more of a midnight snack at 8p if you had to be back up at 4am to milk the cows.
Same for us Swamp Yankees. And because it usually took a couple of hours for our big city relatives to drive - literally drive over the river and through the woods (and up the hills) to our locale, we would have Thanksgiving Dinner around 2pm.
Thanksgiving is by far my favorite holiday, and I have much for which I am thankful. May God bestow blessings on all of us.