fwiw I've had success with the minimalist two-diode type circuit that you linked to. The only difference being that I used the TX pin of the 'sniffer' device as a handy source of -12V to use through a 47K pull-down to make sure the sniffer's own RX didn't float when neither of the snooped-upon devices was transmitting.
Unless you're driving tens of feet of wire, or are running at faster rates like 115.2kbps, the diode & resistor thing shouldn't affect the circuit too much.
If you really want to buffer the signals to TTL and back, there are of course the MAX chips, and even simple old line-driver/line-receiver type chips like the 1488 quad driver and 1489 quad receiver that would do the job.
So in short, it doesn't seem to be a protocol, but instead the name of the transceiver line. Shouldn't be any different from regular RS-232. I guess people like the sound of the name and use it on their products.
Ah, one edit: It seems that the output voltage levels are +/4.2V and it allows full +/-15V input. Some 'RS-232' transceivers don't like the full +/-15V range and only produce 0-5V output. This is a step above those in terms of compatibility with the true RS-232 standard, which is not often completely met nowadays in terms of input/output voltages.
Source (NI RS-232C Standard Conformance)