He is, however, 220 pounds and not known for his rebounding.
Where'd you get that from? All of the reports I've seen of him say he's a good rebounder.
NBADraft.net (I think) had a scouting report saying that he is "not the rebounder he should be with his size". I will probably agree - 8 rpg in college for someone this tall is good, but not extraordinary.
In similar minutes Glen Davis got more rebounds in college as a senior.
Though I'm not sure if pace is similar, or if Davis had other teammates to get defensive boards. (same for Johnson)
College stats don't always translate, though. For instance, BBD also outrebounded David Lee in college, and they played in the same conference.
JJ didn't dominate the boards in college, so it's hard to call him an above-average rebounder at this point, but he could go either way. The athleticism and size is there to be a good rebounder.