You simply posted a dictionary definition, said that canon is not continuity, and then overstated that Star Trek has no real continuity except for "Broad Strokes."
All of which is true.
continuity is certainly a large part of a "canon."
No it isn't, canon and continuity are two separate concepts. The canon of scripture definition has no bearing here, canon status refers to the legitimacy of an entry into a body of work, not whether it is consistent with other such entries.
If you are going to post dictionary definitions, at least give an effort to explain what leads you to make these dramatic absolutes.
I didn't think it needed explaining to be perfectly honest, I thought "nope" followed by a definition which clearly refuted the post I responded to was a fairly clear statement.
When a writer is writing a Star Trek episode, what keeps them to following and rules of the fictional universe?
I'm guessing you mangled two versions of a question here mid edit, so I'll do my best to help you by referring you to someone far more qualified's comment:
I like to think there's a sane middle ground between changing everything every episode and clinging religiously to every last detail from every single episode produced in the last fifty-plus years.
Seriously, any position can be made to sound ridiculous if you take it to an absurd degree.
Writers can't be expected to base every detail of what they make on the minutiae of what someone else has done before, especially when their predecessors never intended for that. Trek is about the ideals first, the details second.