I've guided myself with the PISSORF rule - the more profane interpretaion the more accurate. Maybe it's more obvious to non native speakers. If you pronounce it out loud in a hammy voice (such as when bombasticaly calling out a spell you're casting), and stress every syllable it's pretty obvous. BURNTHEIRASS! BURN/DAYR/AZ!
I think I would have interpreted BURNDEYRAZ as intended.
I also don't think it's particularly important. I mean, most of the time I think: oh yay, finally the fireball glyph shows up.
It's just amusing to me that, while other people were internally screaming BURN DERE ASS in a super deep voice as they flung fire at their enemies, I was lightly sprinkling them with sun beams.