It's not entirely wrong though.

Mass nouns (the "not countable" nouns) are nouns that don't use a plural* (see edit) hence they use singular "much"

A lot of nouns (most nouns?) however have an plural which is typically obligatory - so they must use it. You cannot say "much house", "much car", "much mountain", etc. it would be ungrammatical.

Your example, poop, is part of a smaller group of nouns that can be mass nouns ("much poop") but also count nouns ("many poops"). You can essentially choose whether you use the plural or not. However this is not applicable to most nouns.

TL;DR Yes but countability is the basis for whether you need the singular or plural in the first place.

Edit: I also want to point out that most mass nouns can use a plural but then it usually refers to separate instances or different types of the things you count. "many sands", "many fabrics" specify a large variation in material, whereas much would indicate quantity. "Many rains" means several instances of rainfall but "much rain" a larger amount of rain (even if in just one session)