I'm using mathematical examples since mathematicians like to have very exact definitions.
You can say:
"All even prime numbers are less than 10"
This is true and grammatically correct. The fact that there is only one even prime number is a separate fact and not dependent on the first.
You can say
"All odd perfect numbers are greater than 10⁵⁰⁰⁰".
This is also true. Even though I don't know how many odd perfect numbers exist.
It is even correct to say
"All square prime numbers are negative".
This is certainly grammatically correct and an empty truth, since the set of square prime numbers is empty.
In the definition, "A noun phrase is a noun preceded by all its modifiers", does not imply that there are more than one modifier. It means that each modifier, if it exists precedes the noun. This is not a special use of the plural, but it is standard precise English.