I reckon it’s because I can’t find the basis for you to call his math incorrect… likely because there isn’t a base, only a lack of understanding.
There is no world where a blanket statement of “3iu=1mg” is correct. One is volume, the other is weight, and that ratio varies based on how much liquid mixes with however much ingredient.
I commonly use ranges from 10iu=1mg to 2iu=.6mg. I have some compounded Tirz that is at a 5iu=.9mg ratio… weird, but whatever. I mix at a ratio that is convenient to my dose (which is measured in mg) and keeps the injection volume where I prefer.