The rules-of-thumb about mash thickness were made up back when malt was enzymatically weak. I read that in thinner mashes, the alpha amylase is denatured more quickly than in a thicker mash. This is why old texts recommended to mash thick and hot for a full-bodied beer.
Most modern malts have so much excess enzymatic power (100-120+ DP), it doesn't matter if some, or even half of the enzymes are denatured in a thin mash. There will still be plenty left to fully convert.
I mash super thin now (4-5L/kg, which is about 2-2.4qt/lb), and haven't noticed any difference in the fermentability or body of my beers.