Interesting, personally I'd rather look at DRB% because overall I would say it's more important.Per statmuse.com, mean rebounding percentage for centers this year is 16.8%. He’s 19.1%. It would help to have the standard deviation, but of those who qualify on minutes, there are only 7 guys in the entire league with a higher % (Adams, Jonas V, Sabonis, Robert, Portis, Zubac, Nurkic).
I think Walker has a pretty high level ORB% which is boosting his TRB% a bit.
According to BBALL reference he is 33rd in TRB% this year, not 8th. He doesnt even qualify for the leader board though (based on minutes I guess).