Words like "heap" and "good" are useful even though they have no mathematical precision, but it needs to be understood that these words lack not only mathematical precision but any intrinsic meaning of their own absent reference to the individual items that are being described in summary.
Epicurus is arguing that "pleasure" as a concept has no meaning apart from the individual instances of pleasure which are contained within the summary term. Of course the concept is very useful as a way to communicate ideas, but Plato and the others are asserting that there is an absolute realm of ideas where there is a "perfect" or "form" of pleasure, and that pleasures are pleasures because they somehow mystically partake in this form or idea.
These ideas are overlapping in two threads -- but, here again, I think you nailed it.
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Admin Edit Note 8/11/2025: the other thread referred to, is here.