You could tap the button to add a <strong> element, then type your text and re-tap the button to close </strong>. Or you could select it and tap the button directly, but you'd have to dismiss the OS dialogue. I mean, it looks feasible on iOS, as I don't use Android, I can't really tell…

Presumably like the google drive app, it doesn't show the cut and paste action bar but keeps the formatting bar, it does pop-up a cut and paste dialog.

Evernote does show the cut and paste action bar but also has the text formatting bar at the bottom of the app, which will work on the selection.