Indeed the best evidence supporting this view is that Natanz was fully air-gapped and still got destroyed by the Stuxnet hack.

That said I have heard of customers expressing desire to control valves and pumps using iPhones, and believe there are several initiatives at SCADA/PLC/DCS/System Integrator companies to provide this.

However I've seen as many of those in practice as I have data-diodes, which is to say, none/never.