  We consider $N$ non-interacting fermions in a $2d$ harmonic potential of
trapping frequency $\omega$ and in a rotating frame at angular frequency
$\Omega$, with $0<\omega - \Omega\ll \omega$. At zero temperature, the fermions
are in the non-degenerate lowest Landau level and their positions are in one to
one correspondence with the eigenvalues of an $N\times N$ complex Ginibre
matrix. For large $N$, the fermion density is uniform over the disk of radius
$\sqrt{N}$ centered at the origin and vanishes outside this disk. We compute
exactly, for any finite $N$, the R\'enyi entanglement entropy of order $q$,
$S_q(N,r)$, as well as the cumulants of order $p$, $\langle{N_r^{p}}\rangle_c$,
of the number of fermions $N_r$ in a disk of radius $r$ centered at the origin.
For $N \gg 1$, in the (extended) bulk, i.e., for $0 < r/\sqrt{N} < 1$, we show
that $S_q(N,r)$ is proportional to the number variance ${\rm Var}\,(N_r)$,
despite the non-Gaussian fluctuations of $N_r$. This relation breaks down at
the edge of the fermion density, for $r \approx \sqrt{N}$, where we show
analytically that $S_q(N,r)$ and ${\rm Var}\,(N_r)$ have a different
$r$-dependence.
