Okay, you do realize it can do a couple of hundreds points of damage per round as written?
As well as its three regular attacks with
Multiattack it can make three more with
Legendary Action option of
Attack and it also has that
Incendiary Rebuke reaction.
Its Scimitar does 5d6+7 plus another 2d8 from its
Pillar of Flame (there's nothing to say it can't make its scimitar attacks while a fiery pillar. Indeed, its scimitar is the Noble's only melee attack so what else is it adding the 2d8 to? Also, shouldn't it's pillar also injure creatures near it, not just objects?
So that's six attacks each doing an average of 33½ for a 201 DPR plus a 33 point Rebuke a third of the time for another 11 DPR, for 212 in total.
With an average damage of 212 the CR Calculator says its Challenge 23 not 15!
However, that's not a very good picture because a lot of that damage is fire. Of the 212 DPR, it breaks down to 105 points of slashing from the scimitar and 107 points of fire.
If it mainly uses Hurl Flames rather than its Scimitar the Noble's DPR drops considerably and its damage is ALL fire.
That means a fire-resistant opponent only takes a half or at worst three-quarters damage, while a fire-immune opponent might take no damage at all.
That's more of a bug rather than a feature. The D&D Efreets in Al-Qadim and the like are noted for having trouble fighting each other with their fiery abilities so the nobles resort to all sorts of strategies and deceits to defeat their rivals in the royal courts of the City of Brass.
Might post a counter-proposal for an Efreeti Noble sometime later.
Before I do so, could you confirm what kind of Challenge Rating we're aiming for?