Not that Lion King needed remaking, but I can't call this a reboot in the traditional sense - the Lion Guard is still going after all, and will still be going likely by the point this film airs. By the same criteria, something like Sonic Boom would be a reboot of Segaverse Sonic, despite that both continuities are still ongoing, along with Archiverse continuity, and every other continuity that isn't going anymore, but did in the past.
If the new film is intended as a replacement for the old canon (films, novels, cartoons, etc.), then yes, I'd call it a reboot, but right now, it's more of a remake or "re-imagining." Which is semantics I know, but it's a battle of words that had started before I jumped in. I'd only consider this a reboot if the old continuity came to an end.