The issue here is that there is no single original "Thanksgiving" from which all others descend. Thanksgivings were a common European tradition in the colonial era, and various communities could host one for a variety of reasons.

In 1619, a small party of Virginians held a thanksgiving to celebrate their safe journey across the Atlantic. The Pilgrims famously had a thanksgiving a few years later to celebrate a bountiful harvest - though their Wampanoag allies were not actually invited to the party as commonly portrayed. Instead, the Wampanoag came to investigate the sounds of gunfire coming from the Plymouth colony, which turned out to be celebratory. Seeing that the colonists were having a party of some sort, the Wampanoag warriors killed a few deer and presented them as gifts so they could join in the festivities too.

The colonists also held thanksgivings to commemoriate the end of wars. In New England thanksgivings were held at the end of the Pequot War and at the end of King Philip's War, both of which were devastating to local Native communities. The video doesn't specify which massacre they're attributing to the first thanksgiving, but I'd wager it's the post-Pequot War thanksgiving since it does sort of echo down the years in future New England thanksgivings.

The video also ties modern Thanksgiving to the Dakota War, which is inaccurate. It's true that Lincoln allowed for 38 Dakota men to be executed following the Dakota War, resulting in the largest mass execution in American history. But this was after he pardon 264 other Dakota men that had similarly been convicted. That's neither here nor there, since his Thanksgivings proclamation doesn't have much to do with the Dakota War the year earlier. The creation of a national holiday was more an attempt to bridge the tensions between North and South by giving everyone a common day of celebration.