Just for completeness sake, there's also GPG for iOS as well: https://itunes.apple.com/us/app/opengp-lite/id405279153?mt=8
Fair enough, assuming that Intro breaks the S/MIME support (which I'm guessing that it does) then it adds the risk that users using S/MIME can't verify the integrity of mail in the app. So that's something else that S/MIME users should evaluate when judging the risks.
Do you work at/for/rendered services to LinkedIn in any way?
iOS supports S/MIME.