He didn't even considered fighting Shanks alone, but his whole crew, thats how Shanks strength work.
It keeps being an sneak attack, cuz you're absolutely not expecting it, and if you're not expecting, your reaction responds accordingly
In this case its even worse, cuz there was literally no way of Greenbull to even slightly predict the Red Hair Pirates would show up in Wano, out of nowhere, much less using attacking him from such a long distance.