In the 1920s Furtwängler chose Horenstein as his assistant. Horenstein would prepare, say, the Vienna Philharmonic for for him frequently. No matter what one's overall view of Furtwängler is, he knew infinitely more about conducting than Hurwitz ever could imagine. And in those long ago days starting in 1922 only Bruno Walter and early Mengelberg championed Mahler more than Horenstein.
We have very fine versions of Horenstein's M1, M3, M4, M5, M7, M8, and Das Lied von der Erde that are remarkable. If Mr. Hurwitz doesn't like them, that is his privilege. But his opinion is only his opinion. The recorded legacy stands on its own.