That is strange, the other tutorial link [2] being probably legacy but even the current [3] leverages the syntax masq6='1'
uci set $(uci show firewall | sed -n -e "/\.name='wan'$/s//.masq6='1'/p" | sed -n -e "1p")
kmod-ipt-nat6 or kmod-nf-nat6 should be loaded upon installation but might not have, perhaps check with lsmod | grep nat6. Maybe rebooting the router helps?
[3] https://openwrt.org/docs/guide-user/network/ipv6/ipv6.nat6