He's not elite at offensive rebounds. He get a lot of offensive rebounds because his entire contribution to the offense is hanging around the rim
Do you expect the best offensive rebounders in the current NBA or in NBA history to hang or hung around the three point line, the mid range area or near the foul line? It's kind of a prerequisite to being an elite rebounder of any type that you are near the basket an awful lot.
Now the NBA is littered with and has always been overflowing with big guys that hang around the rim because of limitations elsewhere. So Kanter was fighting not only all those guys for rebounds but also great rebounders who aren't always in the paint. And yet, he still was a top three guy(and the best guy at doing that in two years) for the last five years if you look at offensive rebounding percentage. He took in a higher percentage of available offensive rebounds while he was in the game than anyone else in the league the last 5 years. And that stat is backed up with the stat that, even though he played just 25 minutes a game, he ranked top 6 in total offensive rebounds 3 of the last 5 years.
You can poo pop those numbers with inane justifications all you want, but if you are the best or top 5 in the league at something, especially over a time period of 5 years, that makes you elite at that skill.