Swami Vivekananda just inferred as the original bengali version says :
"mone holo ki jaanis, Sayan i nijer bhashya nije uddhar korte Max Muller hoye jonmechhen,aamaar anek din hoitei oi dhaarana." (Swami-Sishya-Samvad, Sarachchandra Chakraborti, Udbodhan, page 56)
Meaning: Do you know what seemed to me? It was Sayana Himself who was born as Max Mueller to revive his commentaries. This is my "Dhaarana"(impression).
He did NOT make any claim that by using any yogic power He knew that Sayana was born as Max Muller. This was His personal impression or imagination only, which became stronger (vaddhamula) after meeting Max Muller.
But it was not a claim.
Manu writes "Antyaadapi param dharmam" and so learning from mlechchas is not denigrating.