I don't understand your question. It's not like there are 'non' infinity cache versions of these cards floating around somewhere to give you an apple to apple comparison of.
I guess you could interpret their question as being, how important does the Infinity Cache play in the performance of AMD cards? With the way they've laid out their lineup, you really can't compare it because the CU and RA's are drastically different between products. Not only is the IC different by 32MB in the 6750 and 6800, but there's a 20 CU / RA difference as well. I guess your best bet on valuing the performance differences would be to just look at the recent review done by FPS on the RTX 3070 and 6750/6800 AMD cards.
Most of the time, when something is integral to a product and doesn't allow for it to be disabled, you will never know how much or how little it impacts performance.
