That's because the lyrics to mouth music are not intended to really mean anything, they're intended to either teach or replace the fiddle tune. The tune is supposed to be played in such a way that it matches the arrangement of consonant and vowel sounds in the Gaelic song- harder on the consonants, softer on the vowels. It's basically a system of musical notation for an oral tradition. You can actually hear a distinct difference when the same tune is played by a Gaelic traditional fiddler as opposed to someone who learned the tune from the sheet-music.
The other purpose of puirt a beul is to provide music for dancers when no fiddler is available. There isn't always a fiddler handy, but you can throw a ceilidh in your kitchen if you have a few people who know how to dance and a singer who knows some mouth music. That's the way they used to do it on Cape Breton island, and I can hardly think of a better way to spend a long winter's night.
Mary Jane Lamond's presentation of the song is a far cry from a kitchen dance, but that is as it should be- the context is different, and you can't just change the context completely and keep the presentation exactly the same.