Microdosing any substance implies you are consuming less than a standard serving. 1Tbsp, as well as 10 grams, is often a full portion for some.
a micro dose of kava would be more like a teaspoon or a gram or two.
I am sure placebo plays its role and also, this style of kava consumption has benefits. I have likely encountered this situation from brewing multiple washes more than once. Good question!