if you consider electromagnetism an element as opposed to a fundamental force within the universe, that makes sense. most often, elemental refers to things such as earth, water, wind, fire and... lightning. you thought i was gonna say heart, didn't you? and, while magneto can create and absorb lightning, as well as functionally control ferrous earth or anything with trace metals in it, he's not psionically interacting with the elements themselves.
and the meggan v. storm blurb was directed more toward feralboy, just to spark debate. storm has incredible power, i will admit, and she can affect the weather like none other. but meggan empathized with an active volcano and got the lava to stop flowing. her hands even becaming strange streams of lava. and that's when she wasn't particularly aware of what she was doing. if it came to a face-off, it would be hard to judge whether meggan's empathetic link to all things earthen would trump storm's complete dominance of the weather.