It all gets rather complicated because there are two types of iptables, the ‘legacy’ one, and the nftables shim, that actually uses nftables, but looks like iptables. So if you’re using the latter, then LXD will choose to use nftables directly rather than traditional iptables.
See also: Lxd bridge doesn't work with IPv4 and UFW with nftables for tips on getting it to work with the ufw firewall which seems to be quite popular.