Does boiling water deactivate malt enzymes?
Yes, heating to boiling temperature will destroy amylase.
Depending on the ratio, the goal of that recipe may be to destroy the enzymes, to gel the starch, or to help the enzymes be most effective. It’s not uncommon for particularly old and traditional recipes to use a combination of boiling water, ice-cold water, and room-temperature ingredients to reach a particular temperature, as the ratio of inputs will determine the final temperature pretty accurately without a need for a thermometer. If the water to malt/grain ratio is about 1:1 by mass, that would put the enzymes at their optimum temperature for converting starch to sugar. If it’s a lot more water than malt/grain (again, by mass) then the purpose is more likely to deactivate the enzymes and/or gel the starch.