Yup. The city was rebuilt using magic alone. Wouldn’t be too hard for them to lift it into the sky.
They’d only need to move the reconstructed part and the Sunwell.
But so long as the Sunwell’s shield holds there’s no need. There’s even less need for the Alliance to invade since Quel’thalas is cut off from the rest of the Eastern Kingdoms. The Blood Elves can’t move troops by foot due to the plaguelands and it lacks a major sea port. Most travel is done with portals, which they could do from Kalimdor, negating any benefit reason for them to attack.
If the Alliance are stupid enough to waste resources attacking them and they’re able to get passed the shield AND the Sunwell can’t be moved then personally I’d rather see the city evacuated and the Sunwell detonated, taking the Alliance army with it.