App = Information × Interface = User Experience
It's ×(multiplication), not +(plus). Interface is not simply overlaid on Information; it actively changes how information is perceived and used, thus User Experience (UX).
Users have limited screen space, attention time, context retention, etc. So, apps must be wise about what information matters at any given moment, and make the best out of those limited resources.
Developers have been crazy about this: A/B testing, CVR, retention, churn, LTV, ARPU, DAU/MAU, North Star Metrics. Deploy and analyze, develop and optimize, rinse and repeat, and apps end up as revenue-generating machines.
A side effect of this optimization loop is that, apps become a designed thinking process for users. Apps decide what to show, what to hide, what to emphasize, and what comes next. They actively shape how people see and think, all to lure them into spending money.
So, "this could have been a webpage" misses the point of what apps are for, and, by extension, who apps are for.
Still, I see a bit of potential here. Document is a natural user interface -- almost all apps, including even SPAs, have document-like or document-driven views. Perhaps we've been too obsessed with computer-program-like UI. Documents can always be dynamic and interactive without being overdesigned. Perhaps this is what folks wanted to point out.