Mistake in presentation.

Question

Plural uncountable nounles épinardsdesTu manges des épinards.(You eat some spinach.)This explanation is incorrect.  There's no such thing as a plural uncountable noun.  The very definition of a non-count noun is that it doesn't take a plural inflection.  You need to explain this as a difference between what's a count versus non-count noun between the two languages.  "Spinach" is non-count in English but countable in French (hence taking "des."

Accepted Answer

It is not clear from the lesson but épinards is a word that is always used in the plural in French as for example the noun news in English which is also uncountable.
The definition of uncountable is that you cannot count the noun, it does not imply that the noun is a singular noun.
You cannot say: un épinards, deux épinards, trois épinards,... Therefore, épinards is uncountable by definition and the text is correct.
The plural partitive seems to be a point of constant confusion for members based on the already existing posts. The text should point out that the plural partitive only occurs with nouns that are always in the plural (e.g. épinards).
The following link has a list of plural nouns in French:
https://www.lawlessfrench.com/grammar/plural-nouns/