TBH, Stainless heats up slower than cast iron, but usually cast iron is much thicker for cooking so it ends up taking longer (Cast iron has a lower heat capacity than even carbon steel, but a CI pan is usually 3-5x as thick). It's part of why stainless is harder to weld (along with the passive chrome oxide layer, material contamination, etc).
It's just what Napoleon sold. I imagine it's a combination of Stainless being their 'prestige' product lineups normally. I'd prefer stainless for an outdoor BBQ anyway, especially on a BBQ that can go well north of 700 when all the burners are on max. Besides the issue of rusting, I'm not sure I wouldn't butcher the seasoning pretty regularly on a CI griddle of this size in a BBQ.
Upside this is much thicker than I thought. Was worried they'd cheap out a bit and make it thin enough to feel cheap but this thing is sturdy.