It is! They don't really control the elements, it's more that each seraph is aligned with one element and the artes they cast and all their abilities have to do with that... for example, Mikleo is a water seraph, and that means he's really good at making frozen desserts! But Lailah is a fire seraph and she's far better at baked goods.
[So the whole alignment thing extends far further than simply control of the element they have.] I guess it's because the others here don't know about the differences that they simply call it with one name, even if it isn't at all accurate.
[Haaah... while he thinks the real magic is cool, it isn't cool to go calling things magic when they aren't!]