This idea isn't entirely new (is anything?); for an earlier version, check out Klavarskribo: https://en.wikipedia.org/wiki/Klavarskribo. It didn't really take hold, because it's much more inefficient than normal notation.
Someone else suggested that this was a notation with which they were familiar, from an old hymnal of the (U.S.) denomination affiliated with the college I attended. I had assumed it to be quite an old system as the hymnal appeared quite antiquated. Still, it doesn't surprise me that it (only?) dates to the 1930s, nor that there would still be churches using it. Many old Reformed churches have short institutional memories but nonetheless cling to assumed traditions. That, and the Frisians are, stereotypically, an old, stubborn people.