I think the condition for local interface as output interface was for hairpin srcnat rule. Which is usually not necessary, because such traffic won’t go anywhere else anyway. Except when you’d have some overlapping subnets, e.g. for VPN clients who would use addresses from LAN subnet, then this condition would prevent unnecessary srcnat for connections from LAN to VPN clients. It would still happen for connection from VPN clients to LAN, you’d have to stop that using other means (accept rule in srcnat for traffic from VPN subnet).