I'll submit my vote for 'no'. I think that soulfire is not 'fire' any more than gunfire or a fire opal is. Sure, it shares some of the same letters, but it's a convenient conceptualization of something incomprehensible. If you power a fire evocation with soulfire, then I think the immunity to fire would apply (since it's a buffed fire attack, but a fire attack nonetheless). But if you cast a pure soulfire spell or power any other type of non-fire spell with soulfire, then it's not really a 'fire' attack. In fact, the text (YS292) implies without outright stating that it is 'holy' damage.
I'd classify hellfire similarly, by the way (though as 'unholy' rather than 'holy', of course).
(My opinion, for what it's worth.)