Does the game set SO_BROADCAST on the p2p socket? And I assume it tries to send to 255.255.255.255?
Nothing against adding this but you also need track the SO_BROADCAST status(similarly to nbio) of each guest p2p socket and validate if (addr == BROADCAST_ADDRESS && !so_broadcast) { return -SYS_NET_EACCES; }

It does yes. Observed on PlayStation All-Stars Battle Royale. Here's the log:

U sys_net TODO: sys_net_infoctl(cmd=53, arg=*0xd0080018)
W sys_net: sys_net_bnet_socket(family=INET, type=DGRAM-P2P, protocol=IPPROTO_IP)
W sys_net: sys_net_bnet_bind(s=24, addr=*0xd0080204, addrlen=16)
! sys_net: [P2P] Trying to bind 0.0.0.0:3658:1002
! IPv6_layer: is_ipv6_supported(): disabled cause: force-disabled through config
! sys_net: P2P port 3658 was bound!
W sys_net: sys_net_bnet_setsockopt(s=24, level=SYS_NET_SOL_SOCKET, optname=SO_NBIO, optval=*0xd0080200, optlen=4)
W sys_net: optval: 0x00000001
W sys_net: sys_net_bnet_socket(family=INET, type=DGRAM-P2P, protocol=IPPROTO_IP)
W sys_net: sys_net_bnet_bind(s=25, addr=*0xd0080204, addrlen=16)
! sys_net: [P2P] Trying to bind 0.0.0.0:3658:1001
W sys_net: sys_net_bnet_setsockopt(s=25, level=SYS_NET_SOL_SOCKET, optname=SO_NBIO, optval=*0xd0080200, optlen=4)
W sys_net: optval: 0x00000001
W sys_net: sys_net_bnet_setsockopt(s=25, level=SYS_NET_SOL_SOCKET, optname=SO_BROADCAST, optval=*0xd0080200, optlen=4)
W sys_net: optval: 0x00000001
E sys_net: Socket error EACCES
E SYS: 'sys_net_bnet_sendto' failed with 0xfffffff3 : -SYS_NET_EACCES [1]
E sys_net: Socket error EACCES
E SYS: 'sys_net_bnet_sendto' failed with 0xfffffff3 : -SYS_NET_EACCES [2]
E sys_net: Socket error EACCES
E SYS: 'sys_net_bnet_sendto' failed with 0xfffffff3 : -SYS_NET_EACCES [3]
E sys_net: Socket error EACCES x2

And it does send to 255.255.255.255, I've already observed the beahviour while capturing packets in working consoles.

It seems like it would be wiser to implement the socket option in lv2_socket_p2p::setsockopt, the game calls it in the log you displayed.

@Cri4Key Sorry to revive the thread, for PSABR specifically, if I tell another player to set the IP to 255.255.255.255 in Network settings, would we be able to find each other in-game using the tournament mode?

You need to enable write-acces from maintainers, I wanted to merge it but I can't because of that.