“It is very likely that a communication from a client which has 867Mbps rate could easily be “intercepted” or “lowered” by another client which is transmitting 6Mbps on the very same channel that the high-rate client operates. So, in this case, the high rate client or the AP will be affected by this low rate client, but the question is how much?”
Here are the details: Measuring the Effects of WiFi Low Rate Transmissions on the High Rate WiFi Traffic in the Same Frequency – updated Oct 2018
Oguzhan EREN, CWNE#266
oguzhaneren.com 
Nice study, but NOT convinced this is the real world. The slow 20MHz AP is handicapped and only covers 1/4 of the bandwidth. Fast and slow should get the same number of transmission slots, what is not the case here. The fast is using “Dynamic Bandwidth Operation” or air-time fairness is active (https://www.oreilly.com/library/view/80211ac-a-survival/9781449357702/ch03.html). The fast rate should be reduced MUCH more. Normally the Aruba time calculator would explain this, but the used 500bytes A-MPDU is very unrealistic for the 26Mbps , while either 86,8Mbps was used for the fast and a A-MPDU of 512, or 866Mbps was used and a 5120byte A-MPDU size. Unfair comparison !!! It is not 47,6 and 158.8µsec but (4,76 and 158,8) or (47,6 and 1588). But this does not fit your lab test, which is not realistic for 2 AP that cover the same channels.
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Had to translate some Turkisch to English for answering.
Speed x and y give common speed= (x*y)/(x+y) using time calculation.
There is an error in the Aruba frame time calculator. Scenario 2 is ALWAYS using 86,7Mbps in P28 , independent of what is entered in O11 (Payload data rate). Either always only use scenario 1, or correct the spreadsheet, or use this one instead: https://gjermundraaen.com/thewifiairtimecalculator/