As I understand it, alpha amylase can cleave smaller polysaccharides, and eventually take a wort down to fermentable sugars and limit dextrins. But, from what I've read, alpha amylase is not as fast when working on smaller polysaccharides as larger ones.
I think the higher fermentability of wort mashed below 150 has a lot to do with limit dextrinase, which is an enzyme that will break the branch bonds in amylopectin (
http://www.lowoxygenbrewing.com/wp-content/uploads/2016/11/Limit-Dextrinase.pdf). Limit dextrinase denatures at slightly lower temps than does beta amylase, so is active in mashes with a "beta rest," but may have little effect in a mash that starts at higher temps, as it is mostly denatured before any significant gelatinization takes place. By having what is really a "limit dextrinase rest" you reduce the amount of limit dextrins that will be in the final wort, and increase the amount of fermentable sugar.
Brew on