This is a myth that will not die, but
I can assure he was not.
Sonnen was a good, but not great collegiate wrestler. Out of four yeas of eligibility, he made it to the NCAA tournament once, and made it to All American status with an 8th place finish - that is, the lowest spot of which you can be bestowed an All American.
Being a Division I wrestler makes you good, and getting to the NCAA tournament even once is also impressive, but the UFC is populated with highly credentialed wrestlers. Among his peers he isn't anything special in that regard.
In his prime though considering the field of opponents he was in, he was one of the better MMA adapted wrestlers. That is also predicated on good timing, because I don't think there is any version of Sonnen that beats any version of guys like Weidman and Romero.