To try to be optimistic you could have a new chain that is on the tighter end of new tolerance and it might get a bit better as it wears.
I recently had slippage on my campag when I replaced the chain. I rode carefully while I waited for the new cassette and by the time it arrived it was all working perfectly. I could barely get the chain measurer on the new chain.
FWIW I don’t think mileage is an accurate reflection of how hard a chain has had to work – conditions, power, lube and cleaning have a huge impact as well as the quality of the original chain. SRAM might be different and higher quality contril but I have a sneaking suspicion that not all Shimano XTR chains are created equal.
Can you trust yourself to remember not to stand in the 42?