> What is the objective measure of usability to which you are referring? I'm not aware of any such metric.

Evaluators normally use scoring systems for this purpose, in which heuristic violations are rated on a severity scale (commonly 0-4). So when apps like Obsidian rack up many 3s and 4s, you have an objective basis on which to characterize it as unusable. Besides this, there are accessibility and security metrics consisting of pass/fail tests, which are countable.

I guess the best analogy would be a street vendor passing dog meat off as beef. You may think the dog is delicious, but that doesn't make it beef.