You are using the wrong method to remove elements from the set. discard removes an element from the set only if it exists. You want to remove elements based on a condition, so you need to use a different approach. Here's a corrected version of the code:

a = {'ab', 'z x c v b n m k l j h g f f d s a a', 'q w e r t y u i o p'}
a = {x for x in a if len(x.split()) >= 9}
print(a)

This code creates a new set with only the elements that meet the condition, and then assigns it back to a. The desired output is achieved:

{'z x c v b n m k l j h g f f d s a a', 'q w e r t y u i o p'}

You already answered the question. It refers to sets of sets (actually sets containing frozensets).

The paragraph you are referring to begins with:

Note, the elem argument to the __contains__(), remove(), and discard() methods may be a set.

which means that b in a.remove(b) can be a set, and then continues with:

To support searching for an equivalent frozenset, the elem set is temporarily mutated during the search and then restored. During the search, the elem set should not be read or mutated since it does not have a meaningful value.

which means that if b is a set, a.remove(b) will scan a for a frozenset equivalent to b and remove it (or throw a KeyError if it doesn't exist).