I have no idea about it. Others said that every coset has a unique representative of the form $f(X)+g(x)Y+h(X)Y^2$, and then it is easy to see that $x \in \mathbb{C}[x,y]/x^3+y^3-1$ is irreducible. But I still don't understand it.
I hope you can give some suggestions to help me understand it or show other ways to prove it. Thanks in advance.