Again - HE said that, not me!
Do I think he played his best tennis during his mature years? Yes and no.
Obviously, his movement was not on par with his peak years but the easiness of his movement and his anticipation and experience vastly compensated for that. His FH was not as potent as earlier years, but again, not tremendously off. Now, his BH was considerably better later on than in the past, his slices too, his serve evolved in arguably the best serve on tour in later stages. His net game was MUCH more effective in the later stages of his career not only because it shortened the points but also because in that era, there were not many players that knew how to handle him on the net.
As for your last comment - that describes the whole RF “dominance” - it was easy to dominate Hewitt, Baghdadis, Phillapousis, Roddick, sidetracked Safin, very young ND, very old and crippled AA, Gonzales, but not fully developed ND, Murray, streaky Delpo, wawrinka, Tsonga and solid Nadal (when healthy).
By the way, RF losses during that 2012 season were mainly to people I listed above, when he had true, solid competition.
In fact, he came back SIX years later and won another slam!