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.
Of course, we as white people so often have to make it about us, about our inability to empathize with the concerns of those who experience the daily realities of discrimination and the perpetual threat of real violence at the hands of the police. We pretend we know better. Worse, we pretend we know what's best for black Americans. We minimize their protest, their pain. We try to drown out their voices with our own.
But it's not about us! It's not about white people's discomfort with racial inequity. It's not about papering over black anguish so we can sleep better at night with our opinions and our ideologies and the votes we've chosen to cast. "All lives matter" has been and always will be a deflection used to reorient the conversation away from contending with systemic racism. And it's not the first of its kind. You could fill a coffee table book with white American turns of phrase that were specifically designed to wrest the conversation away from the disenfranchised. You could title it 101 Ways to Avoid Talking About Racism.
Now, is Grant Napear a racist? I don't know. I suspect not. Is he prone to very incendiary language? Yes. Has he throughout his career wielded a number of racist "dog whistles" when discussing particular Kings players of color, particularly those who don't fit the comfortable, squeaky clean mold that a lot of sports fans want to try and project onto black athletes? Also yes.
Like so many Americans, I'd guess that Grant doesn't even realize he's participating in rather odious traditions of racist invective and paternalism when his blood is up and he starts aiming that vitriol at black players. His ignorance, if it is ignorance, doesn't excuse his behavior. To the contrary, I am glad that he's finally being held accountable for it. Media figures should be held to a higher standard of speech. They have a much larger platform than you or I. They must be more responsible and conscientious than you or I might be when we decide to air out our thoughts and/or grievances online or elsewhere.
A couple dozen Kings fans might read this post that I've written. It doesn't amount to much (and may even be deleted, which is the KF.com moderators' right if they deem I've overstepped KF.com rules). But Grant Napear had thousands of listeners. And far too many of them are coming to his defense rather than to the defense of those whose legitimate protests are being threatened by the boot heels and batons of militarized police forces all across the nation. That is the impact of a phrase like "All lives matter."