That coating is not conductive. So he is correct that the switch won’t self-ground. However, you can remove that coating with a dremel or other roto-tool. It will take about a minute.
Also, unless there is something weird in Canada, there shouldn’t be any reason you can’t physically ground the switch if you don’t want to grind the coating. The “never do it” is probably they usually don’t do it because the devices self-ground. At least in the US, we simply ground the switch with a pigtail. The metal box will have a screw hole. Insert a grounding (green) screw and attach a length of bare copper. I can’t imagine how this would be against code, but I don’t know the CA code. Pigtailing a ground should take about 2 minutes. (And this is exactly how it’s done if you have to use a plastic spacer on a metal box.)
TBH, grinding off the coating ought to get it. Either way, no reason not the by Inovelli because of the coating.