I tried to make sense of the obd-2.com website to see what cable you are referring to, but started to get a headache. That site is a nightmare. Let me try and explain the RS-232 to ISO-9141 interface issue:
* RS-232 from your PC signals with +/- 12V (although the voltages seem to get smaller and smaller these days as chipmakers cut corners and nobody seems to care about RS-232 much anymore).
* RS-232 uses "full duplex" where there is a seperate wire for each of the two parties to talk on.
* ISO-9141 from you car uses the +12V supply from you car and signals betwen +12 and 0 V.
* ISO-9141 is "half duplex" where all parties communicate on one wire by taking turns talking and connecting to the wire electrically in a way that if you are not talking, others can talk (open collector signalling in this case).
You need some small amount of simple electronics to interface between these two systems - they are not electrically equivalent. There are ways you can do it without optocouplers, but optos are just as easy to do and provide some extra margin of safety against damage to the ECU or PC if there are problems.
I chose to make a USB based serial interface to elimate the problems that some laptops have with low RS-232 signaling voltages and to allow the interface to operate at obscure/arbitrary baud rates that most PC UARTs don't support. Many newer laptops don't have serial ports anymore either.