I don't think this is correct. For years, the #3 search query on Bing in the US was "Google", and globally it used to be a double-digit percentage of all Bing queries. That suggests to me that people with a default Bing search engine had learned in droves to click their way to the preferred engine regardless of what the default was, and did so without being technically skilled enough to change the default once and for all. I don't know how large a group the latter is, but it seems hard to argue that the two together are small.