The whole idea of “serial” versus “null-modem” cables as you describe them is a holdover from RS-232 connections as used in almost all non-classic-Mac computers. RS-232 defines connections in terms of a “data terminal” and “data set” — i.e. a computer end and a modem/printer/whatever end. All of the signals are named with respect to the computer end and the wires go from each pin at one end to the SAME pins at the other end (this is the plain “serial” style of cable). This means that the Transmit pin is still called that even at the far end where it is receiving. Obviously, if you tried to connect two Data Sets or two Data Terminals together with a cable like that, both ends would be trying to transmit data into the same wire, which would not be connected to any receivers. The solution to that is to use a special cable with one end wired up backwards; this is a null modem cable, so called because as far as the directly-connected computers (Data Terminals) are concerned, it works as though it were standing in for two modems and a telephone connection.

RS-422, the standard loosely adhered to by classic Macs, is defined differently. EVERY RS-422 socket, whether on a printer, or a modem, or on the back of the computer, is supposed to be wired up the same way, so all RS-422 serial cables are technically null-modem cables. The fact that Apple’s own printer engineers initially got confused about that and did it backwards, RS-232 style, has left us a legacy of two different kinds of Mac serial cables that look identical but work only in specific circumstances.