Earlier quoted context omitted.
>Smartphones in their current form cannot have secure messaging. What about self-hosted matrix/element, used from the browser?
Maybe but 1) You don't get push notifications on most mobile OSes 2) Mobile users have poor control over their browser (if any.)
And I'm not sure what you're referring to by "control", but Firefox on Android allows you to install addons and use about:config (Chromium can't do the former and doesn't have an equivalent for the latter, even on desktop).