What does the array_filter key in $form do?
Nice catch! It does nothing. You can safely remove it. Looks like some kind of forgotten relict.
Ah, yeah. The same pattern can be found in core's book module: BookSettingsForm.php.
And there is a 12(!) years old open issue to have it removed. [#315176] Clean-up remains of $form['array_filter'] hack with array_filter in book module