"This wasn't a mistake, it's been common knowledge."
Knowledge of a mistake doesn't mean that it didn't originate as a mistake. It did take a long time to get corrected, yes, and many people knew of this being an example of non-free software existing in Trisquel. It's still a mistake though because non-free software doesn't belong in Trisquel. Whether you want to call the inclusion a mistake or an error or whatever the outcome is still the same.
