That isn't true, but it isn't far off.
From 2014-2017 (4 seasons), JD Martinez ranks seventh among all qualified hitters in MLB in OPS. His .936 OPS trails only Trout, Votto, Goldschmidt, Stanton, Harper and Big Papi. If you only look at 2015-2017, his OPS is a little higher (.943), but he still trails the same six names. (Apparently Papi was still considered a qualified hitter over that stretch.)
Not second, but still impressive. Especially for a guy who was once released from a last place team while still making minimum wage...