But Sybok himself his never referenced in the rest of Trek, so what's really keeping him tied into the cannon besides a ridiculous movie? It's just like, all of a sudden Spock has a goofy brother and then that's it.
Almost everything else TFF brings to the cannon of the series is in small references to outside facts, not so much anything that has to do with the plot of the movie in a major way.
Sybok is part of the Star Trek canon because he was seen in a movie. That is all that is required. This ridiculous movie may be the only thing keeping him tied to continuity but he is and forever will be part of the Star Trek canon.
You're really confusing canon with continuity. Canon is simply a body of work. period. In the case of Star Trek canon is that which is seen on both the large and small screens.
Continuity is totally different and yes there are many things in the entire franchise that contradicts earlier facts and therefore doesn't fit in with continuity.
Here is an exampe: If the next movie depicts Vulcans with yellow blood it will go against the continuity that Vulcans have green blood but it will become part of the official canon of Star Trek because it is part of the body of work that is seen on either the small or large screen.
Canon: the body of rules, principles, or standards accepted as axiomatic and universally binding in a field of study or
art.
Continuous: being in immediate connection or spatial relationship.