There’s the Legends Expanded Universe, and now there’s the canon Expanded Universe.
If it’s not in the movies, it’s still EU, canon or not.
Well, if you want to get technical, “Expanded Universe” is a term that was created to describe specific Star Wars content. It’s a term that has now been retired, with LFL referring to all content now as canon (or universe) content. Obviously anyone is free to decide what is or isn’t canon in their own minds. And obviously people are allowed to use the term EU if they want. But it isn’t a phrase that is used anymore, and while one could argue that term or not, nothing’s changed, that’s not exactly true, as there is at least an effort being made to keeping an internally consistent universe (whereas when Lucas was in charge he didn’t care even a little). More importantly, in part because of this, the lines are being blurred. You can say anything that’s not the movies is EU, but what about Rogue One and Solo? There’s a lot more interconectivity now too between all mediums.
That being said, there’s definitely some wisdom to your kind of mindset. When it comes to the telling of a story that is explicitly a sequel to another story (rather than just another story in the same universe), what should be considered important to that story is what was important in the preceding story. In this case, just because one can point towards Ackbar’s expanded role in other content doesn’t mean he was an important character in the episodic saga that TLJ is a part of. On the flip side, non episodes could probably stand to learn that lesson (I’m thinking about stuff like Vader in Rogue One). If all of Star Wars content is supposed to stand on its own as content in and of itself rather than expanded material, they should try to use the films and their characters as a crutch a bit less.