You're assuming you're fed and the varus is an idiot. You can replace fizz with any champion + fed and anybody + idiot and your comment would still make sense. And under the same logic, ezreal or any other adc with a jump would be equally fucked against this apparently impossible to react to fizz, despite the fact you have a good second to react to an e in, and the invulnerability only lasts as long as you stay balanced on the pole and pressing e again instantly cancels it, making you waste most of your invulnerability for mobility to jump into a full team, just to possibly q the adc once before you get cc locked and instantly cleansed the instant your r hits the adc, if at all.
Varus is strong. There's no real argument of, oh, but an assassin can jump him and kill him. Well duh, that's what an assassin is supposed to do against any adc, regardless of any mobility the adc has because an assassin is a burst caster with high mobility by definition. Their only counter is lack of vision and hard cc. Varus has the latter, and that gives him the ability to fight an assassin on somewhat better footing than somebody like ezreal, who can get out maneuvered. Varus just ults and ends the mobility advantage of the assassin and puts himself into the damage and burst advantage because of the lockdown and his blighted arrows.