There's an even more prosaic reason for it to be Ra and not Nightmist - the Ra deck is being designed by Fred Hicks instead of Eric Vogel. Fred knows Sentinels Ra much, much better than he knows Sentinels Nightmist. (This was asked and answered at the DFCO Kickstarter page.)
See that's why I keep coming back to these forums. You ask a question, and you get answers that make sense.
Also for what it's worth Dresden is a Fire Blasty Wizard himeself, so Ra migh just make sense as a fun pairing.
I wanted to back this but other things claimed my money.