While Campari is quite bitter, it works well in this recipe going up against the Lemon Juice and, more importantly, the Honey Syrup. Using regular Simple Syrup likely wouldn't add enough sweetness here nor give the right flavour up against the bitterness of the Campari. You can also tell from the recipe that the portion of Honey Syrup is pretty generous! If things do come out too sweet for you, or you just like things bitter, feel free to dial back the Honey Syrup a little bit. Ultimately you're making a cocktail for you, so make it how you like it!