I think it's meant to cover a body of holy men acting in concert, rather than individual shamen, priests and what-have-you within warbands; the Battle of Chester is pretty much 'the' example. Despite the blurb that pretty much excludes all but Christian monks.
Limiting it to Britons, Welsh and Irish, who were all solidly Christian (if not the right kind of Christians) throughout the era, seems legitimate at first glance, but it does exclude King/Saint Oswald's Northumbrians, who are likely to have had them at Heavenfield.
Given the scarcity of solid sources in the era, hard and fast special rules are a little dubious in their application. I would have no issue with more liberal use of all of them across all armies, with the odd exception.