I don't think it necessarily matters, actually.

This is an unfortunate limitation of relying solely or even primarily on native speakers as linguistic authorities. They are certainly the best sources of information as regards what terms and phrases are actively used in a language, as well as how these are properly used, and how they should be pronounced. They cannot always be expected to know why a term or phrase is used, however, or how it originated.

This is the case in any language. Even in English there are plenty of phrases like "play it by air", which have widely used variants "play it by ear" or even "play it by year". All native speakers understand the intended semantic meaning of any of these phrases is the same; to respond to a situation as it develops. Native English speakers will naturally struggle to suggest how it developed etymologically, however. Depending on the variant used by any individual speaker, they may suggest it has something to do with playing an instrument without referencing sheet music (by ear/air), or doing things differently one year to another (by year).

It strikes me that the same thing is probably the case here, and that even if one native speaker had an idea what the etymology is, it may have been derived by working backwards from a homonymous variant rather than by knowing the true origin of the phrase. If that is the case, one native speaker may have one explanation of its origin, and another may have a different one altogether, so in a case like this it may not be worthwhile relying on an individual native speaker's knowledge at any rate, unless you're going to poll a large number of them all at once.

I had an interesting experience years ago in Muskerry, after the bean a' tí said "toast" was arán tíora. I couldn't work out what this was, and she didn't know. Arán tíre - bread of the country? Arán tuíre - bread of hardness? Eventually I realised it was aran tíortha, scorched bread, with the "th" not always pronounced....