Show $f[x_0,x_1,\ldots,x_n]=1$ for divided differences of $x^n$

As kindly suggested by Patrick Da Silva, I'm turning my comment into an answer.

Let $x_0,x_1,\dots$ be distinct real numbers, let $f$ be a polynomial function on $\mathbb R$, and define $f[x_0,\dots,x_j]$ for $j=0,1,\dots$ recursively by $$ f[x_0]:=f(x_0), $$ $$ f[x_0,\dots,x_j]:=\frac{f[x_1,\dots,x_j]-f[x_0,\dots,x_{j-1}]}{x_j-x_0}\quad,\quad j\ge1. $$

It is straightforward to check that $f[x_0,\dots,x_j]$ is a homogeneous polynomial of degree $d-j$ in $x_0,\dots,x_j$ if $f$ is homogeneous of degree $d$.

In your case, $f[x_0,\dots,x_n]$ is constant, and the statement follows from equality $$ \lim_{(x_0,\dots,x_n)\to(\xi,\dots,\xi)} f[x_0,\dots,x_n] = \frac{f^{(n)}(\xi)}{n!} $$ in Wikipedia.