Again, you are confusing the recording process with the mastering process. Once the instruments/tracks have been recorded they can be changed completely independently of any other instrument/track. In the mastering process all those recorded tracks have been mixed together and cannot be unmixed. However, there may still be the possibility of processing certain individual elements. Cymbals for example produce high freq transients so a gate to a processor could be opened just for the HF and just for the duration of those transients and although you would still be processing all the HF in mix, the cymbal transients will dominate over all/most other elements in the mix for those instants when the gate is open, effectively giving the ability to process the cymbals independently. There are a number of routinely used mastering tricks/techniques available to effectively isolate and process individual or limited numbers of elements in the mix. Which of those tricks/techniques MQA employs and how intelligently we don't yet know.
The main point I was addressing however was your repeated assertion that EQ is the only process which can be applied during mastering/re-mastering. That assertion is incorrect and, it's certainly not "simply misinformed" to point out that it's incorrect, it was "simply misinformed" to make that assertion in the first place!
G