No way Jetti would give you any other answer than NO. Their lawyers would shoot them.
Did the source of this suggestion have first hand experience with running these at this voltage?
5S at 4.2V is 21V. Do not use nominal voltage for this calculation. 6S rated ESC typically list 25.2V max. These limits are not made up, but real limits of components incorporated into the design. Exceeding voltage limit = dead components, period. It's a fairly hard limit(there are tolerances) unlike a current limit which is more about heat dissipation.
The other issue is that if you don't charge to full voltage you certainly won't get full capacity out. To exceed useful watt-hours of a 5S fully charged with a 6S, it will have to be charged to over 83%. Considering the 6S will start at lower voltage it will actually have to be more. Looking at the cell spec sheet, you won't get that if you only charge to 4V.
Does anyone use drone ESC? They are typically quite small, but current ratings need to be taken with a grain of salt because they are usually installed right under props or at least in the open air.
https://www.amazon.com/gp/product/B0CDGFX1QS/ I have a couple of these for an RC app but haven't even tested yet.