RTF is a global modification that replaces all towns in the game.

As far as I know, "Sudden Storm" won't work if the unit doesn't have the "Storm Strike" skill, and if the unit is associated with the element of wind, it would probably be more logical to give it vulnerability to the opposite element (earth) rather than immunity.

Well, it seems to me that it should be vulnerable to fire and water, but immune to earth and air, and "Sudden Storm" should work without Storm Strike, because it was present in Storm Titans, and I also saw a mod where it worked normally without Storm Strike.