Anatoly Karpov's comment that he and Bobby Fischer were stronger than Magnus Carlsen is rather hard to believe and is almost comical, but I'll offer four remarks in his defense.
First, he prefaced it with "I think", offering a bit of a hedge. He wasn't making a categorical pronouncement.
Second, Karpov is assuming that rating inflation is obvious. Given that assumption, his supposition becomes more plausible.
Third, he notes that Carlsen is still developing. Though Karpov, like Carlsen, became the world champion in his early 20s, he didn't reach his peak in his early 20s but sometime later (in fact, his all-time highest rating was accomplished when he was 43 and his highest official rating when he was 45!). So Carlsen has plenty of time to improve even further.
Fourth, Karpov's claims may be based in part on dominance, and both he and Fischer had longer and/or clearer margins of dominance than Carlsen.
In reply, the first rebuttal makes it easier to swallow but doesn't do anything to support the claim on its merits.
Point two has been disputed by Ken Regan, who claims that there hasn't been rating inflation. (There was a brief period where there were maybe 30 points' worth of inflation, but that bump was subsequently eliminated.) In correspondence and conversation I've asked whether his model fails to take the increased depth of theoretical preparation into account, and in reply he has noted that even if we compare the players of today with those of yesteryear taking only moves 17-32 into account, there's still no good evidence of rating inflation.
Point three, like point one, mitigates the shock value of the claim but doesn't support the claim itself.
Point four is both iffy and a change of subject. Fischer's lead over the rest of the world was greater than Carlsen's, but Carlsen's lead was greater than Karpov's when the latter became champion. Karpov was then dominant for years, but as Carlsen only won the title last year the time hasn't elapsed to make the comparison of their reigns. And even if Karpov's reign proves more impressive than Carlsen's, relatively speaking, it doesn't show that he was the stronger player. Emanuel Lasker was great and was world champion for 27 years, but I don't believe that Karpov concludes that Lasker was therefore the strongest player of all time.
Anyway, it's an interesting interview, and there are other entertaining bits to be savored and disputed as well.