In many cases it isn't.
Arguable the question title is not counting anything
"is ... a quantifier"doesn't count anything.
The argument that there's an aspect of existence fails, because the aspect exists even without a determiner, eg. before uncountable nouns. If it is the default, it does not need mention. However, young children who have not learned object permanence would disagree, believing only in what they are seeing (peekaboo). So you might need a cognitive linguist to answer.
Another example, taken from German, fails in direct translation. Apparently the article is required: "I have a thirst". This sounds strange, because construction with I am is preferred. A strong preference for tangible objects with have can be observed. But it leads to the strange result that money is suddenly uncountable. The count just doesn't exist, for what it's worth, if it's none of your business.
That's still different from is a. If not a(n) meant "not even one", then the quantificantion is apparent. Indeed, wiktionary derives an from the same root as one. Also cp. no one, nobody.
However, translations of no one derive from deictic particles so frequently (in Greek, Kurdish, Hindu, German, neque) that this probably represents an original state of affairs, and it's a reasonable assumption that a(n) as well reflected a similar one, at least in part (cp. e.g. the derivation of it, or the inclusive particle in). Logicly you answer not "an", but "yes" (from one such determiner *ge-) to questions of existence, although you might answer with a precise wuantity. Although, it would be an etymologic fallacy to give too much weight to that. Vice versa, this question is interesting for the reconstruction.
All that aside, one could interpret the title as "part of the set", "of the type", for which the existence of the set is prerequisite, not introduced by the determiner.
I'd consider that distributive, because it can also be something else at the same time.