Did you check if it is actually not reachable over IPv4? IPv6 socketscan also accept IPv4 connections unless configured not to do so. This is controlled system-wide via /proc/sys/net/ipv6/bindv6only which is "0" onmy computer (which is the default according to man:systemd.socket(5)).
And for some reason I don't know this is set to 1 here. Setting it to 0 makes it work. Sorry about that.
