Thabo is also 31 years old and may only have a couple more good seasons in him while Ben is 22 and hasn't even entered his prime yet. And while I agree that Thabo is clearly the better defender, comparing dRtg across teams is always problematic. I always look at the player relative to the rest of their team instead and I find that to be a more reliable means of comparison.
SO with Thabo Sefolosha, his team ranges from 98 to 106 and he's near the top at 101. That's very good. With Ben McLemore, the Kings range from 103 to 113 (Moreland hasn't played enough minutes to count) and he's at 110 which is near the bottom. If Ben were to be traded to Atlanta and maintain his relative position he's probably at about 105 on that team so the comparison is 101 to 105 not 101 to 110. And if Thabo were traded to Sacramento and maintained his relative position he would be at about 106 so the comparison there is 106 to 110. Either way it's about the same. Thabo is a better defender, but not by the huge margin that the raw numbers imply.