Folk etymology is pretty much defined by people thinking of an origin for a word or phrase and saying, hey, that’s sounds good to me. And then they’re done. They don’t have to do any research, don’t have to check whether anybody ever used it that way, don’t have to refute any other claims. That’s why nobody takes it seriously.
When you do check, you find that girdle string - usually two words, not one - does exist as a phrase. In fact, it refers to something that is an exact and obvious predecessor to a g-string.
Google books has numerous examples of girdle string. Check The northern tribes of central Australia By Sir Baldwin Spencer.
The Mara tribe man wears pubic tassels that dangle from a girdle string. A picture is given. Women wear an apron that is slightly wider.
Waist bands, thin strings referred to as girdles, are found in numerous cultures. Material culture depicted in Vijayanagara temples By K. Reddeppa. That’s exactly what the first reference to a geestring Cecil found was.
It’s quite old, too. Here’s a reference from 1846. The living age …, Volume 9 By Eliakim Littell, Robert S. Littell, Making of America Project from a section titled “Elephant Shooting in Ceylon.”
I don’t find geestring earlier than the John Hanson Beadle book Cecil referred to (though the date is given as 1877). G string is contemporary.
Catholic world, Volume 35 By Paulist Fathers, 1882
That’s exactly the same as the girdle-string above.
For an etymology, it would seem almost certain that g string or gee string came from girdle string. No references of any kind provide any evidence that it came from a g-string on a violin or other other musical instrument. Nor are there any references to indicate its origin in garter or groin.
You don’t look at a word and imagine what it could derive from. You have to patiently sift through all the actual references in the writing that survive to tease out its meaning. Folk etymology is looking at a blurred dot on a photo and concluding that it must be an alien ship.