It seems to be impossible to open any link inside an article using Switch Control in the iOS app. Executing the "Tap" action on any link does not open it but instead it just resets the Switch Control position to the "<Explore" back button in the navigation bar.
Switch Control is an iOS assistive technology for severely paralyzed people or people suffering similar disabilities who cannot control the device in traditional ways and must rely on control by a single switch or a small number of switches (e.g. pressing a switch by palm of hand, activating a switch by winking or tilting their head to a side).
Therefore, users relying on Switch Control won't be able to browse Wikipedia by following links in articles.
I made a test using a simple demo app with just the navigation bar, tab bar and a UIWebView with a Wikipedia article open in it and Switch Control did open links normally, so there must be something in the iOS app that prevents it from doing so.