I'm curious to know what that means. Mahler very clearly specifies that timpani need to be 'pedal tuned', and that suspended cymbals are to be 'free hanging', etc. Nowhere does he state that horns need to be single F or single Bb, or that trumpets have to be of the rotary valve variety with their faster 'German' tapers to the bell sections. Neither does he say that silver flutes are verboten, or that Clarinets have to have the 'Albert' system to their keys. Maybe unwound, 'gut' strings would make some difference, but certainly baroque style bows were already long gone. If you want to be truly authentic, then both Paiste and Wuhan tam-tams would need to be excluded.
What about bass trombones that have the cone shaped 'Thayer' valves on them? Should those be excluded as well?
Perhaps more to the point, nowhere in Mahler's writing does he express a yearning to hear orchestras as they used to sound in some sort of imaginary 'golden' era of more natural sounds. Like Berlioz, Mahler was very much a 'forward thinker' when it came to matters of orchestration.
I'm not calling such an en-devour a waste of time - I'm just curious as to what it specifically and objectively means.