Yes, the singers are not as good if we had a Ferrier, Forrester, or Baker, but we have to remember that any performance of Mahler in 1924 did not attract participation by the premiere vocalists of that time, for Mahler's acceptance as a composer still had a very long way to go. Consequently, experience in performing the Mahler idiom was absolutely minimal at that time, at least as far as singers were concerned, not to mention orchestral players. And yes, by contemporary recording standards, any acoustical recording of any Mahler symphony is bound to sound deficient, which is why I asked for comment on the performance, not the sonics. I read somewhere that when the recording was made, there were around fifty(?) or less performers total crowded in front of a huge recording horn. You can hear various bass-range instruments substituting for others, such as tubas for string basses, as the recording apparatus was unable to pick them up. For a conductor who likely seemed dedicated to performing the work as close as possible to reflect the composer's intentions under unbelievable conditions as these is short of miraculous.