I'm not sure if your post will also survive given that the post you're responding to was deleted, but I think there are a couple things that can be stated, sticking to the facts as much as possible.
First, given what Jason Jones and others have said, Grant's departure was about more than just that Tweet. Whether due to racial insensitivity or not, he was an alienating enough figure that current players refused to come on his show. That would appear to be a sufficient business justification for termination in itself, no?
Second, the Tweet itself is problematic. As you suggest, some interpret "Black Lives Matter," to mean "Black Lives matter only," and not "Black Lives Matter, too." But, as you also note, the first interpretation is at odds with what the protestors mean. So, why not listen to them! And, given all that the Kings organization itself has done with BLM the past couple years, not knowing that the second interpretation is what really matters is itself problematic, because it makes it look like he wasn't paying attention to what the team and players were saying. Thus, Grant's claim of ignorance actually hurts his case. Either he knew how his response would come off, or he should have known, and either way it's not a good look.