I'm pretty sure the error is when they later take the input and eval it, and the author's managed to dodge their filtering rather than execute arbitrary code in the context of an array-to-string cast (which I was lead to believe when reading that post, at least). Otherwise it implies that some permutation of:

$a = '{${phpinfo()}}'; $b = [$a]; $c = "$b";

Will execute phpinfo()... which it won't.