One potential problem here is that this only applies to page blocking interstitials that promote an app. Page blocking interstitials that push signing up for a newsletter, for example, will not be penalized.
It could be hard to argue that an interstitial pushing an app (especially an app specifically for the site the user is visiting that gives them enhanced features compared to the mobile web version of the site) is more annoying, distracting, or otherwise harmful to a user than any other kind of blocking interstitial.
Thus, one might try to conclude that Google's actual interest here is trying to keep users from using apps to access content instead of using the web site where Google is more likely to be the ad provider.
I have no idea if this could rise to the level of an antitrust violation, but it's the first place I'd look for one.
There's an alternative that is more user-friendly which the browser provides, but only for app install interstitials. Since there's no built-in alternative for other kinds of interstitials, it makes sense that they're left alone (although the crappier your site, the less time people spend on it, and presumably the farther it falls down the search results).
Also, the blog post states they're essentially calling these sites mobile-unfriendly, which I think is entirely fair. All full-page interstitials are annoying, on the mobile web or desktop web, but only "install our app" interstitials are unique to mobile. So if you view "mobile friendly" as meaning "as good as desktop web", banning bad things that are only relevant to mobile makes sense.
Yeah, I dunno. I could buy the argument that a modal on launch isn't always evil (maybe it's a signin prompt), so we don't want to just block that kind of UI outright; we instead want some heuristics to tell us whether the modal is helpful or not, and prompts to download apps are almost always unhelpful.
Still, I feel like newsletter prompts are also easy to detect, so that argument doesn't hold a ton of water unless there's also a separate plan to address those—or maybe they're testing the waters with this one. I think I need more information to have a real opinion :/
It could be hard to argue that an interstitial pushing an app (especially an app specifically for the site the user is visiting that gives them enhanced features compared to the mobile web version of the site) is more annoying, distracting, or otherwise harmful to a user than any other kind of blocking interstitial.
Thus, one might try to conclude that Google's actual interest here is trying to keep users from using apps to access content instead of using the web site where Google is more likely to be the ad provider.
I have no idea if this could rise to the level of an antitrust violation, but it's the first place I'd look for one.