Then again with how ELO works it doesn’t necessary mean that it was Magnus’s peak only that that the point when the gap between him and the rest of the chess world was the largest. I think others became stronger and he had more competition. Still Magnus himself seems to think he is past his peak in interviews.
I mean, he himself wouldn't necessarily be able to tell the difference of him getting worse or the rest getting better. To him it's just getting harder to beat people. At his level how can you judge yourself unless you played against a fixed-version AI chess program?