I don’t know if the author is making the distinction, but Plymouth and Massachusetts Bay were separate colonies until 1691. So, it’s possible that Plymouth had their thanksgiving feast with the native people in 1621, while Massachusetts Bay Colony had it’s first in 1630/31.
Or, the distinction may be being made between the celebratory harvest feast of the Pilgrims and an “official” thanksgiving feast. The Pilgrims didn’t see their multi-day party with the natives as an official thanksgiving feast at the time, but as a celebration of the harvest. There was a set religious protocol to follow for a “real” Thanksgiving that they did not engage in. It may be that the author is saying that the first of these official, religious based thanksgivings that the Puritans would have viewed as a such didn’t actually happen until 1630/31 in the Massachusetts Bay Colony. Thanksgivings in Puritan society could be called for several different reasons, and not always after the harvest.
This website might be helpful:
https://www.plimoth.org/learn/multimedia-reference-library/read-articles-and-writings/thanksgiving-history
