His original description sounds simple. Doubling delta time will effectively just double the speed, will double the distance traveled in one frame and will fail horribly if things are determined on a "snapshot" base. If enemy x starts the movement outside a towers range and ends the movement behind its range, the tower won't fire at it in the first place. Bullet speed never comes into play.
Keeping the simple approach can work, as long as all speeds and sizes/ranges are carefully designed and in combination with fixed time steps. It will still be inconsistent. Assuming the time step allows an enemy to move half a tower radius, it might start shooting anywhere from the enemy just barely having entered and the enemy already being halfway through. Not even fixing time steps can help with this (though aligning everything in a grid of carefully selected size could).
Personally I'd prefer to go with the extra math: you don't just check "is the enemy in range now", but "did it move through my range this update, at which point did it enter, when did the tower start shooting, where exactly did bullets hit the enemy during its movement, etc.". I like to believe that treating movement as stuff actually moving, rather than just teleporting around is somewhat important to have "proper" physics.
Of course, sometimes "simple and kind of good enough" is all you got time for...
