That is not a direct quote. Your semi-quote "but diversity also makes things come apart for a large secular nation like the United States" is inaccurate and extremely misleading; Haidt clearly ends the sentence at "come apart", and you left off the "And so" starting the next sentence.
The actual quote, starting at 9:20, with as much fidelity as I can muster:
"We have this idea that umm... that diversity is good, and diversity has many good effects. But.. diversity also makes things come apart. And so for a large secular nation like the United States you have to look at what are the forces holding us together, what are the things blowing us apart. Now diversity makes [a] group more creative when you have good norms, when things are well structured. We have to think really carefully about how to get the benefit from America's diversity, but it's hard to do because if you critique it you could get in big trouble. Now in terms of what actually holds a country together, traditionally it's shared gods, shared blood, and shared enemies. That's what nations usually have used. So we have a challenge, and it is a great experiment, and when social media came in -- when everybody was on social media beginning around 2012, 2013 when it gets hyperviralized, umm, the ability to have any shared understanding of what we're doing shatters. Social media allows us to participate in microstories that kind of bubble up and are gone. There is no ability to have a common understanding of what we're doing. Not that we ever were all one nation and all on the same page. But there's a qualitative change when it's like... here's the story of the day, and.. and.. umm.. uhh.. so there's no possibility for shared stories in the age of social media, widely shared stories. Umm... uh... there's huge decline of trust, trust in each other and trust in institutions. And here I'm drawing on recent social science - uh, political science research showing that social media generally leads to a decline of trust. Social media's incredibly powerful for tearing things down. And that can be a good thing in a dictatorship. But it's very bad at building things up. And in an ailing democracy like ours where our institutions need to be improved not ripped apart, it generally has... has made things worse."
