In the case of most startups, native apps are unnecessary. I already see that feeling trend my geek friends ("Ah, not another of those sites that want me to download their app. I just want to see the [movie listings, items for sale, opening hours, etc.]").
Worth noting, also, that Facebook (and Twitter) is mostly just a window with a browser (hence why the mobile sites are very similar).
I think the point the original post was trying to make was that in deciding between Web and native app, only native apps will be necessary. In other words, why spend the time developing a Web app when everyone will use it on a mobile anyway.
(I just want to point out that there's a large part of my soul that is just torn over agreeing with this post, because I would much prefer an open Web to proprietary apps. And I built Web apps for years. However, denial does not become an entrepreneur.)
That, and an app seems to have more authority than a website. "This is Facebook, and I know it." sort of thing.
I discovered it when I was at Starbucks. My session expired and the NewsFeed displayed the Starbucks login page instead.
And from what I understand, Facebook has done a lot of significant engineering to try and improve the performance while keeping it mostly HTML5.
Mobile web apps aren't ready.
Pure speculation, but I wouldn't be surprised.