Sure. E.g., AppKit on OS X is in general not thread-safe (still), and for many years apps were almost exclusively single-threaded. But apps and frameworks cou

Sure. E.g., AppKit on OS X is in general not thread-safe (still), and for many years apps were almost exclusively single-threaded. But apps and frameworks could occasionally spin up threads as long as they were guaranteed to do work in some bounded set of functions (i.e., as long as they were sure not to call into AppKit to update the UI...). So while the author's example is a bit contrived, I can see where it came from.