You haven't seen Maximum Overdrive then. You fortunate, fortunate person.
Stephen King is a filmmaker because he does have one professional credit as a director. [0] The debate remains that Stephen King is probably not a good filmmaker.
amateur versus professional: King was paid so he's a professional filmmaker.
bad/unskilled versus good/talented: King showed himself to be a aesthetically bad/unskilled filmmaker.
You are offering a third axis:
"one-off" versus "career": King so far has not made a career of filmmaking and only made the one film.
In English yeah, we do tend to assume the first axis (amateur versus professional) as the "default" axis. I think this is a very useful axis to use as the default: it's the least gatekeeping and the least subjective. Have you been paid anything to do that job? Congratulations, you are a professional at it. You've done the job. Otherwise, you are an amateur, keep trying you'll get there some day.
"one-off"/"career" is an axis with more subjective judgments. (How many films does it take to call it your career? 2? 14? If you deeply and academically study films your whole life but only make one masterpiece, is that not a career effort?) I shouldn't need to explain how good/bad, skilled/unskilled are deeply subjective.
The problem is that films are not books (shocking, I know) and what works in one medium of storytelling doesn't necessarily work in the other. As in actual translation, there has to be some liberty in how something is adapted from one form into another or else it will be very accurate but not very good.