In this sutta he tells the Brahmin about the problem that the Brahmin teachers never seen Brahma face to face.
But then he mentioned he saw face to face the universe.
He say he knows the Path to Union with Brahma. And goes to explain the Buddhist Path. He talks about the true practice of a Brahman. In many suttas. So obviously he preached the correct Path.
Think also all the instances the Bodhisatta was portrayed as a Brahmin. Or as an ascetic following Brahmayana.
See Verses of the Elders. They mention mastery of the Vedas.
So I see the tradition they had but we didn’t understand it.
Remember Bodhisatta became a monk under Buddha Kassapa as a Brahmin.
And there is more.