Just some comments from fans on the inaudible nature of metal lyrics...
And it's commonly understood that the inability to understand metal music and particularly its vocals, is the norm because the genre employs unintelligible harsh vocals, low-pitched registers, and distorted sounds.
There are plenty of other genres that audibility is challenging. Just take giants of the business Tom Petty, Bob Dylan, or Geddy Lee for instance. And we won't even mention the intentional confusion like of songs like Sussudio, lol.
Sure, with sufficient listening, you eventually can make out what's being said. But across the board, and I've listened to scores of genres of music, taking moral bias out of the equation, there is no doubt in my mind that metal is the most difficult to hear what's being said lyrically.
So their stated intention for using their platform to advance the gospel seems at least have some self-inflicted cognitive dissonance or discord.