For fiction, the term "canon" means "that body of work that is accepted to exist". By definition, if it isn't canon, it is not accepted to exist. That which is not canon may still be interesting, and may still be sold, but that doesn't mean it can be used as a measure of what actually exists in the fictional universe.
So, the Rakata? Not canon. The Jed'ii? Not canon. The Star Forge? Not canon. These things, though written about, are not accepted to actually exist. They are all.. speculative.
And, say, the Yuuzhan Vong? Totally not canon at this point. They do not exist. Jacen Solo? Definitely does not exist. Han and Leia had one kid in canon - Ben Solo. Ben borrows heavily from Jacen, but they are not the same.
There is a whole lot of Star Wars stories that do not exist.
Canon is HIGHLY debatable.
In many instances Canon relies upon the original creator's consent to actually be official. In that light, the old EU is actually MORE Canon than the Disney stuff. It's not that Lucas wrote it or even said it was his own story, but that he at least approved of it being written and at times actually input specific ideas and decisions he wanted done with the over all storyline of the EU. (for example, he specified that they could not kill Jacen which led to Annakin Solo's death early on, and I suspect the entire Jacen plot overall had some relation to Lucas's meddling/input to the EU later on, though it was very limited in his input).
With Disney, they tossed any ideas he had input with the EU with the exceptions of the things that he had wished to be put into the Clone Wars. Disney really hasn't adhered to the "original creators" wishes and thus, in line with what is normally Original Canon, Disney really doesn't have it.
Thus, in reality, Lucas has his specific Canon, much of which we never saw. Then we have Canon of which he had a little input, but overall is NOT his, but he approves of it as an alternate universe or story to his. It is thus still an original, but lesser type of Canon (something similar would be the stories of DS9's later seasons and Voyager for Star Trek in relation to Roddenberry).
What they have would be Disney Canon or their companies Canon. This is not unusual and follows many properties. We see it with Conan, we've seen it with the Terminator, we see it with the MCU and many other properties. This type of Canon seems to change far more often depending on the whims of whomever owns the property at the time or is in control of the property. Sometimes it lasts a long time, sometimes it just has a reboot. The EU and Lucas's personal Canon are NOT part of Disney's canon. What is Canon from Lucas seems to be the 6 movies and parts of the Clone Wars animated series. I'm not sure if Lucas's Ewok movies are even Canon (I suspect they are not), or other small items Lucas has said over the years are also not Canon anymore, despite Lucas being the source of SW and those items.
So, just because it's not part of DISNEY Canon does not mean it is NOT Canon, it simply is not part of the Canon that Disney is endorsing currently (and that type of stuff changes on the fly, even in the short time Disney has owned SW, the Canon has changed a bit with some things being non-canon to suddenly being Canon and vice versa...it's shifting...much like a LOT of these market owned Canon things do).
Conan is a great parallel and example. Conan has had different Movie Universes (at least two) that have different stories to them, comics, and other things which are Canon to the specific companies that make the different types. These Canons have changed in various ways over the decades depending on who owns or is running the show.
On the otherhand, the actual stuff written by REH has basically stayed the same (probably because he is dead and is no longer writing any of it). Even with some new additions (pertaining to discovered documents which he wrote) it has added on and stayed fairly consistent.
With Conan you see several different types of Canon...however...ultimately it is up to the reader or one being entertained to determine which Canon they feel they wish to follow.
Normally, all things being equal though, it is the ORIGINAL CREATOR's Canon that is the baseline, and his approved additional sources (such as what Lovecraft did) can sometimes be seen as a secondary base...with the additional Canon's being unofficial in the long run and only followed by those who wish to follow that specific Canon.
