Well, that's one hypothesis. However since they saw a significant difference in the population where the social media apps were removed, then if your hypothesis is true, the data would suggest that delivering the service as a native app rather than a web app must have some harmful effect in itself. An alternative hypothesis is that their application usage patterns changed when the apps were removed.
It's not altogether far fetched that web-delivered apps have a different psychological effect than native smartphone apps, because native smartphone apps have greater access to the system for tracking and notifications. Native apps also offer different features than their web versions. This is why I use Facebook via a browser, because the Facebook native app is insufferably intrusive, constantly trying to get your attention. It means, however, I can't use Facebook's chat function.