$\newcommand{\ZZ}{{\mathcal Z}_G}
\newcommand{\NN}{{\mathcal N}_G}
\newcommand{\zz}{{\mathfrak z}_G}
\newcommand{\Lie}{{\rm Lie\,}}
\renewcommand{\tt}{{\mathfrak t}}
\renewcommand{\gg}{{\mathfrak g}}
\newcommand{\X}{{\sf X}}
\newcommand{\Z}{{\Bbb Z}}$
**Yes,** this is true for any $n\ge 3$.

Let $G$ be a semisimple group *of adjoint type* 
over an algebraically closed field $k$ of characteristic 0.
Let $T\subset G$ be a maximal torus. 
Write 
$$E=T^{(2)}:= \{t\in T(k)\ |\ t^2=1\}.$$
We wish to compute the centralizer $\ZZ(E)$ and the normalizer $\NN(E)$.

Observe that  $\NN(E)\supseteq \ZZ(E)\supseteq T$. 
We compute  $\zz(E):=\Lie \ZZ(E)$.

> **Lemma 1.** $\zz(E)=\tt:=\Lie T$.

*Proof.*
Since $\ZZ(E)\supseteq T$, we have $\zz(E)\supseteq\tt$.
Write the root decomposition
$$ \Lie G=\tt\oplus\bigoplus_{\beta\in R}\gg_\beta\,,$$
where $R=R(G,T)\subset \X^*(T)$ is the root system.
Since $\zz(E)\supseteq\tt$, we have
$$ \zz(E)=\tt\oplus\bigoplus_{\beta\in M}\gg_\beta$$
for some subset $M\subseteq R$.
Here
$$M=\{\beta\in R\ |\ \beta(t)=1\ \forall t\in E\}.$$
Let $S\subset R$ be a system of simple roots (a basis of $R$).
Since $G$ is of adjoint type, the set $S\subset R\subset \X^*(T)$
is a basis of the character group $\X^*(T)$ of $T$.
It follows that for any simple root $\alpha\in S$,
there exists $t\in E=T^{(2)}$ such that $\alpha(t)=-1$.
Let $W=W(G,T)=\NN(T)/T$ be the Weyl group.
The group $W$ acts on $E$ and on $R$, and $W\cdot S=R$.
Therefore, for any root $\beta\in R$, there exists $t\in E$
such that $\beta(t)=-1$, and therefore
$M=\varnothing$ and $\zz(E)=\tt$, as required.

We compute $\NN(E)$. 
It follows from Lemma 1 that the identity component  $\ZZ(E)^0$ of $\ZZ(E)$ is $T$.
We see that  $\NN(E)$ normalizes $\ZZ(E)$ and $\ZZ(E)^0=T$, 
that is, $\NN(E)\subseteq \NN(T)$. 
On the other hand, $\NN(T)$ normalizes $T$ and $E$, 
whence $\NN(T)\subseteq \NN(E)$.
Thus $\NN(E)=\NN(T)$.

We wish to compute $\ZZ(E)$.
Consider the Weyl group  $W=\NN(T)/T$.
Set $$W_E=\ZZ(E)/T\subseteq \NN(T)/T=W.$$
Then $W_E$ is a finite group, the kernel of the natural homomorphism
$W\to{\rm Aut\,} E$,
and $\ZZ(E)$ is the preimage of $W_E$ in $\NN(T)$.

> **Lemma 2.** Let $W'=S_n$ (the permutation group on $n$ symbols) 
naturally acting on 
$$E'=\ker\, \Sigma\colon (\Z/2\Z)^n\to \Z/2\Z.$$
If $n\ge 3$, then this action is effective (has trivial kernel).

*Proof.* Left to OP and the reader.

Observe that Lemma 2 is false for $n=2$, when ${\rm Aut\,} E'=\{1\}$
whereas $S_2\neq \{1\}$.

> **Theorem.** Let  $G={\rm PGL}_n$ with $n\ge 3$.
Then $\NN(E)/\ZZ(E)=\NN(T)/T=W$.

*Proof.* 
We have seen that $\NN(E)=\NN(T)$. By Lemma 2, 
if $n\ge 3$, then $W_E=\{1\}$, whence $\ZZ(E)=T$, and the theorem follows.