How could CMN600 route snoop transactions to RN-F

TgtID isn't required on the SNP channel as once the flit leaves the RN-F interface, there's only one possible target for it - the RN-F that is connected to that interface.  However, that doesn't prevent the interconnect from using a TgtID equivalent internally.

As with all AMBA protocols, CHI defines how the external interfaces of devices should behave, but does not define or limit the internal functionality of the device.  An interconnect might have a CHI external interface, but internally could use any protocol that it wished.  It might use CHI internally with extra signals, or it might use something completely different.