I think Clare is exaggerating somewhat. The melody "Old Hundredth" is in the public domain. If I create an abc file or .gif file or .midi file containing nothing but the melody Old 100th then anyone may copy the melody from my abc file or .gif or .midi file. Copyright only extends to what is original. If I change the melody, then copyright covers my changes; but then the melody is no longer Old 100th, it's my adaptation of old 100th.
If I create an arragement with original harmonization, instrumentation, whatever, then the copyright covers the harmonization, instrumentation, or whatever. If I incorporate the melody with my arrangements in, say, a .midi file, then supposedly copyright covers the .midi file in the sense that no one may copy it entire without permission. But one is still permitted to transcribe the public domain melody itself (apart from my harmonization and so forth) by listening to the midi, or to use a software tool to reverse-engineer the midi to notation, extract the public domain melody, then delete the copyrighted stuff.
I don't see how an abc file of a strictly public domain melody can sustain a copyright in the melody (as opposed to any notes or comments that might be attached) any more than conventional musical notation of a public domain melody could sustain a copyright.
The question comes down to whether the melody is or is not in the public domain. Most so-called "traditional" melodies probably are, but there are pitfalls and complications. A melody might be an original composition yet escape to the oral tradition and there be mistaken for a folk song, as happened to "Silent Night" for a while. The flip side of this is when people falsely claim to be the authors of folk songs, saying that it was their own composition which escaped to the oral tradition against their wishes. This happened with "Home on the Range", though in that case the claim was eventually disproved after a months-long investigation.
As usual on the web, this post is strictly private opinion. Nothing in this post is legal advice or establishes a lawyer-client relationship (I'm not even a lawyer).