The meaning, clear by original context, of the author, head at instagram, would be "web data [as opposed to [total user web &] app usage data]". Which matches closer to what he's responsible for in parallel.
But, continuing technical distinctions outside of original context, in web jargon, web would be HTML served over HTTP (preferably even on www subdomains), and, by my single-utterance litmus test: "browser-viewable". These apps do make content available over/on the web, but having the distinction available seems worthwhile, web traffic versus app traffic. Many apps don't. Some of the content is meaningfully "not on the web" by the same common sense appeal.
> There is no way this is the case as we have many times as many daily users as they have total signups. Maybe this data is web data?
[0] https://www.threads.net/@mosseri/post/DCkAjj1y5qz