Here is a (surprising) proof using Cauchy-Schwarz and "rearrangement".
The following lemma will be the key.

**Lemma**
: Let $X,Y$ be independent integer-valued rvs, then \begin{align*}
(a)\; &\mbox{ for any } z: \;\mathbb{P}(X+Y=z)^2\leq \big(\sum_x\mathbb{P}(X=x)^2\big)\,\big(\sum_y \mathbb{P}(Y=y)^2\big)\\
(b)\; &\sum_z\mathbb{P}(X-Y=z)^2=\sum_z\mathbb{P}(X+Y=z)^2\end{align*}
**Proof**: (a) apply Cauchy-Schwarz to
$\mathbb{P}(X+Y=z)=\sum_x \mathbb{P}(X=x)\mathbb{P}(Y=z-x)$

(b) let $(X^\prime,Y^\prime)$ be distributed as $(X,Y)$, and independent of $(X,Y)$. Then
\begin{align*}
\sum_z\mathbb{P}(X-Y=z)^2=\mathbb{P}(X-Y=X^\prime-Y^\prime)=\mathbb{P}(X-X^\prime=Y-Y^\prime)\\
\sum_z\mathbb{P}(X+Y=z)^2=\mathbb{P}(X+Y=X^\prime+Y^\prime)=\mathbb{P}(X-X^\prime=Y^\prime-Y)
\end{align*}
Since $Y-Y^\prime$ and $Y^\prime-Y$ are identically distributed, and independent of $(X,X^\prime)$ the right hand sides above are equal. **End Proof**

Now to your question above. Let $n=2m$. We have to show that
\begin{align*} \mathbb{P}(X_k+Y_{2m-k}=\ell)\leq \mathbb{P}(X_m+Y_m=m)\end{align*}For the right hand side above we have (using 1. below)
\begin{align*} \mathbb{P}(X_m+Y_m=m)=\mathbb{P}(X_m=X_m^\prime)=\sum_l \mathbb{P}(X_m=l)^2\end{align*}
To transform the left hand side, observe that well known properties of the binomial distribution give:
\begin{align} 1.\; &Y_k \mbox{ is distributed as}\; k-X^\prime_k, \mbox{where $X_k^\prime$ is distributed as $X_k$,}\\&\mbox{ and independent of $X_k$}\\
2.\; &\mbox{ $X_{m+j}$ resp. $Y_{m+j}$ are distributed as $X_m+X_j$ resp. $Y_m+Y_j$, where}\\
&\mbox{ the summands are independent}
\end{align}
Using 1. gives that $\sum_l \mathbb{P}(X_m=l)^2=\sum_l \mathbb{P}(Y_m=l)^2$ and further that
$$ \mathbb{P}(X_k+Y_{2m-k}=\ell)=\mathbb{P}(X_{2m-k}+Y_k=2m-\ell)\;,$$ so we may w.l.o.g. assume that $k\leq m$.
Using 1. and 2. we have
$$\mathbb{P}(X_k+Y_{2m-k}=\ell)=\mathbb{P}(X_k-X_{m-k}+Y_m=\ell+k-m)$$
where $X_k,X_{m-k}$ and $Y_m$ on the right hand side are independent.

Using part (a) of the lemma (with $X=X_{k}-X_{m-k}$ and $Y=Y_m$)
gives $$\mathbb{P}(X_k+Y_{2m-k}=z)^2 \leq \big(\sum_x \mathbb{P}(X_k-X_{m-k}=x)^2\big)\big(\sum_y\mathbb{P}(Y_m=y)^2\big)$$
Finally using part (b) of the lemma on the first factor gives
$$\sum_{x}\mathbb{P}(X_k-X_{m-k}=x)^2=\sum_x\mathbb{P}(X_k+X_{m-k}=x)^2=\sum_x\mathbb{P}(X_m=x)^2$$
so that ultimately
$$\mathbb{P}(X_k+Y_{n-k}=z)^2\leq \big(\sum_x \mathbb{P}(X_m=x)^2\big)\big(\sum_y\mathbb{P}(Y_m=y)^2\big)=\big(\sum_x \mathbb{P}(X_m=x)^2\big)^2\;,$$
as desired.