I somehow assume that the interconnect between the M1 Ultra dies boils down to bunch of AHB ports more or less directly connected together through the interpose

I somehow assume that the interconnect between the M1 Ultra dies boils down to bunch of AHB ports more or less directly connected together through the interposer (with only minimal pad logic). This seems consistent with both the performance profile of the thing and with macOS mostly ignoring the topology for scheduling purposes.