Is Myhill-Nerode equivalence class of a language which contains all palindrome pairwise distinct?
Your proof goes also through with a larger alphabet. Let $x,y\in \Sigma^*$, with $x\neq y$. We use as $z$ the word $10^{|x|+|y|+2}1x^{rev}$. Clearly, $xz$ is a palindrome and therefore the word is in the language. The word $yz$ however is not a palindrome. Note that the middle of $yz$ there has to be a $0$ coming from $z$, hence for being a palindrome the middle of $yz$ has to be centered between the two $1$s of $z$. As a consequence, $yz$ is a palindrome only if $x=y$. Thus $yz$ is not in the language.

For the proof you only need that $|\Sigma|\ge 2$. I would say having more letters makes it even easier, since you have more possibilities to choose the split word $z$.