It is not "pseude SLIP", it would be the real thing.
SLIP is just a framing protocol to transmit IP over a serial
line. It doesn't have to be sending these packets to an internet
provider to make it SLIP. If each machine has an IP address, and
is routing packets to the other machine's IP address via the slip
interface, then they will talk IP over the line. They don't even
need modems, if you are local!
> If the waiting modem take the call and connect, can the incoming
> packet be passed to the socket with a dummy IP a bit like
> it's done in TIA?
No. There is no dummy IP involved. Each machine has a real IP
address (though you may have to just make them up), and sends IP
packets to the other.
Alan