There's always more that can be said, but this topic came up in another thread today. First, there's this summary from /u/mikedash:

This is the assertion made by the project leader, the NYT journalist Nikole Hannah-Jones, in her introductory essay, that the American Revolution was not a war fought in the name of liberation and freedom, but, rather, one undertaken to ensure that the institution of slavery survived at a time when it was already possible to fear that sentiment in Britain was becoming increasingly abolitionist. (Hannah-Jones caused further upset by suggesting that Nazi Germany based its racial policies on those of the contemporary United States, but this controversy was drowned out by the far greater one caused by her remarks about the Revolution.)

The signatories of a critique of the Project that the Princeton professor Sean Willentz circulated to the press have three key objections to Hannah-Jones's thesis:

• They say that it's too cynical – that it offers a dark vision of an America that has made much less progress than most Americans think. “It is this profound pessimism about white America,” The Atlantic pointed out in a story on the controversy, “that many of the 1619 Project’s critics find most galling.”
• They say that it exaggerates the significance of slavery to the decision of the American colonists to rebel – Hannah-Jones suggests that “one of the primary reasons the colonists decided to declare their independence from Britain was because they wanted to protect the institution of slavery,” specifically from rising abolitionist sentiment in Britain.
• They say it over-stresses how systemic racism is in the US – which may in turn cause political paralysis and promote the belief nothing can be done about the problem. There were also objections to the idea that the progress that has been made is fragile and potentially reversible.

There has been quite a bit of to-and-fro about this; supporters of the 1619 Project have argued that its critics are wedded to an ethnocentric, ahistorical and indeed Whig vision of American history as a triumphal progress towards ever greater freedom and ever greater institutional perfection. But the current state of play is that Hannah-Jones has rowed back her position on the causes of the Revolution somewhat – holding that her overall views are correct, but conceding they were too firmly phrased – while Willentz, who has maintained his critical position, nonetheless professes himself in broad support of the overall aims of the project, at least insofar as they relate to the benefits of imaginatively re-focusing and re-periodising American history.

I'd make three points about this dispute. First, there are broad areas of agreement between the Project and its critics – most obviously, both sides agree that America has been shaped by slavery and its legacy, and that racism still shapes American society. It's generally accepted that the 1619 Project has the potential to help tackle the still fairly pervasive influence of the discredited Dunning School (named after the Columbia historian William Archibald Dunning (1857-1922), who at portrayed Reconstruction as a period of tragedy, characterised by “scandalous misrule of the carpet-baggers and negroes”) among the general public.

And I offer this context related to the statement you're asking about (bolding mine):

Hannah-Jones' introductory essay, published in August 2019 can be read via the Wayback Machine read:

Conveniently left out of our founding mythology is the fact that one of the primary reasons the colonists decided to declare their independence from Britain was because they wanted to protect the institution of slavery.

In a December 2019 piece about the project in the The Atlantic, Adam Serwer wrote (again, bolding is mine):

Hannah-Jones hasn’t budged from her conviction that slavery helped fuel the Revolution. “I do still back up that claim,” she told me last week—before Silverstein’s rebuttal was published—although she says she phrased it too strongly in her essay, in a way that might mislead readers into thinking that support for slavery was universal. “I think someone reading that would assume that this was the case: all 13 colonies and most people involved. And I accept that criticism, for sure.” She said that as the 1619 Project is expanded into a history curriculum and published in book form, the text will be changed to make sure claims are properly contextualized.

In a March 2020 update to the piece, the New York Times editor offered:

A passage has been adjusted to make clear that a desire to protect slavery was among the motivations of some of the colonists who fought the Revolutionary War, not among the motivations of all of them. Read more.

It now reads.

Conveniently left out of our founding mythology is the fact that one of the primary reasons some of the colonists decided to declare their independence from Britain was because they wanted to protect the institution of slavery.

Which is to say, Hannah-Jones recognized she went too hard in that paragraph and offered a correction. The "read more" link contains multiple connections to sources that support her claim which, in effect, went from "one of the reasons for the American Revolution was a need to maintain chattel slavery which Great Britain was moving away from" to "some of those who supported the American Revolution did so because they were worried changes in Great Britain could put chattel slavery at risk."