A Tails developer responds in the next message on the link you posted. Pretty much, if someone roots your copy of Tails, they can disable the firewall anyway, by either shutting down iptables or flushing the rules. Even worse, they can copy all your super secret files like your PGP private keys. I haven't seen the script, but I accept the explanation that it makes certain firewall operations easier for someone who wants muck around with Tails as root. However, an adversary with root privileges can fuck you way harder than that script can.