[Discussion] Why do you think Natsu couldn't eat this fire?

In Natsu's brief encounter with Lecka, why do you think Natsu couldn't eat Lecka's fire?

I know one of the popular theories is that he's a Devil Slayer. But when he was casting his spells, he began them with Fire-Make, whereas Gray; the Maker Mage who is a Devil Slayer, does create things with his Devil Slayer Magic, but the spell names always differentiate which of the two Magics.

So do you think it still is Devil Slayer Magic and there's a reason for the above "discrepancy" (for lack of a better word)? Or do you think it's something else? Would love to hear people's thoughts and theories!