To be fair, if anything it implies that fire has more potential than magma. Like in real life, magma temperarure is in general much higher than regular flame temperature, but there can be flames approaching if not surpassing magma temperature
So Oda saying things like King's flames being like magma shows just like in real life, that fire can potential burn as much if not more than magma
And I can't help but feel it might be a setup for the mera mera no mi Sabo ate for Ace, where at one point, he might end up channeling fire surpassing Akainu's magma