I think that folks need to slow down a bit with the hard conclusions here. As I said earlier, he's not a "lock" at this point. But his case for the HoF is probably going to be a lot stronger than some I think are saying. And it's not as if his career is anywhere near over yet.
For one thing, if we remember that this is the Basketball (not NBA) Hall of Fame, then it is important to note that his two NCAA championships, where he was the best player on that team, probably carries some significant weight with some committee members.
Second, while Horford has never been a big 'stats' player, he nevertheless has been very consistent and durable and in particular, has managed to get to the playoffs all but one season of his career and has played rather well when he's been on that stage.
Assuming he gets to at least 3-4 more playoff runs before he retires (something that seems _very_ achievable) and acrues just his career per-season-average numbers in each appearance, Al will likely retire in the top 30 all time in playoff rebounds, top 20 in playoff defensive rebounds, top 50 in playoff assists. If he gets deep playoff runs similar to or deeper than these last two years (18 & 19 games), then he'll blow past those thresholds. That will put him in the company of some very big names.
No guarantee any of that happens, of course. Injuries happen, etc., etc. But if healthy and the Cs do as well as we all hope, then he likely hits those marks pretty easily. Combine that with his NCAA record and his incredibly strong 'respect' cache around the league and I think he'll have no problem getting in. Especially if he gets a title.
But all that just points out that his career is far from over and no, he's not a "lock". But he's almost certainly a much stronger candidate than any numeric model like the bb-ref predictor implies.