Well I tried to find some data to back up my claim that Zeller just doesn't get to the same shots in the paint that Horford does, but to be honest I couldn't. I was looking at the player tracking data on nba.com, and for the guys who have played at least 20 games this year and had at least 5 FGA in the paint, Zeller's actually in the top 10 in opponents' FGA at the rim. So if anything he's getting to more... I'm not exactly sure what to make of that, because all the top 10 guys played 24 minutes or less, and there's many other variables I didn't take into account.
I still think just looking at FG% at the rim is a bit unfair because guys face different types of attempts at the rim (I also noticed a few guys like Timofey Mozgov completely changed in this area when they changed teams), but I can' really make any claim based on stats that Horford is a better rim protector than Zeller (I still think he's better from anecdotal evidence).