Sam, I disagree with you, especially on your comparison.
The inyan of being dan l’kaf z’chus doesn’t negate the inyan of vih’yisem n’kiyim, so despite the fact that you properly assume that someone on a smartphone in shul is using it as a siddur, it might still be the wrong thing to do.
I don’t know where you daven, but where I daven, most people use siddurim, so no assumption is likely to be made that someone is reading a book.
I have most definitely seen people using a smartphone for non-davening purposes in shul, so in my experience, that’s a much more reasonable assumption to make.
I think it’s worth the twenty second investment of time to grab a paper version from the shelf.
Of course, I might be biased, because I’m a traditionalist at heart.