Ultimately, I agree that Kopitar was the better defensive player. What I meant (but didn't really explain) is the voting for the Selke was different during Modano's prime.
During Modano's prime, generally speaking, the voters picked lower scoring 2nd and 3rd line players as the Selke winners/finalists (Lehtinen, Peca, Madden, Draper, Walz, Ricci, Handzus, Arvedson, McCauley etc). Yes, there were exceptions (Forsberg, Sakic) but generally the voters seemed to focus on the forwards who scored less, and really focused on defense. During Kopitar's prime, it felt like the voters were looking for two-way forwards (who played great defense, but also scored a fair bit - Bergeron, Datsyuk, Toews, Richards, Kesler, Couturier, O'Reilly, Stone, Barkov, etc).
I think that if they switched eras, Modano's voting record for the Selke trophy would have improved, and Kopitar's would have gotten worse, simply due to changes in what the voters look for.