Did you read that part about there not being decay in moist clay soils
if buried in a sheet?
I fail to see the sheet/non-decay connection. Seems to me it is a
matter of lack of drainage, which creates an anaerobic condition, plus
the lack of soil microorganisms. Dead soil that won't rot dead bodies.
Certain clays are impermeable and do not allow the passage of water.
That's why they are used to seal ponds.