Google has been saying this from a long time and I agree with them. Interstitials are not a good experience for users. Google made this statement in the past based on a case study () of how Google+ used App Interstitials to promote users to download the Google+ app. And later they decided to drop the App Interstitial because it didn't lead to any significant improvement in app downloads, but did lead to a much higher web page abandoned rate.
Google has already added a point about interstitials (in 2015) in the Google Search Console usability errors report (refer to the snapshot ). Off course it was simply saying that your mobile usability errors would increase but now it's a part of mobile search algorithm.
Besides this I feel social media websites like Facebook should also take actions on sites that have interstitials that ask user to take specific actions when accessed from the Facebook default browser.
Jim Munro: Hi ;, thank you for your participation here. This one belongs in our SEO News community as try to reserve this channel for SEO questions. Please use this link for news items.