For example, operating an engine across the entire mixture spectrum has been a fact of science since the invention of the internal combustion engine. This is not new. It is not a theory. It is a fact of science, provable by any investigator--which has been done thousands of times.
Walter this may seem trivial to you but for scientists and engineers 'theories' are models that have been proven to work, as opposed to an unproven hypothesis. A "scientific fact" is an observation, but if a model gives a useful approximation reliably, then its validity can be considered a 'fact'. You have already had this explained to you on this thread. Conflating a hypothesis with a theory is most definitely not a mistake that a scientist would make.
This video explains it:
the scientific method.
Please note #3, of the so-called 10 commandments: "Thou shalt back up thy statements with evidence...just claiming something is a fact doesn't make it a fact".