TRRS is generally stereo output mono input, yes.
My comment here has more information in the specific context of the internal audio on the FW13:
I vaguely recall that a number of years ago, some Macs had switchable 3.5mm audio ports, such that, I think, you could set the interface, for example, to have TRS stereo input and no output, and set line-in vs mic-in; these also had optical input/output on the same port; at the time Apple was still trying to cater to creative fields and a featureful audio interface was a selling point. But a feature like this would require specific support from the chipset (switching between input and output on the same line). At least with the internal FW13 interface, that support isn’t there.
In general, my understanding is that the 3.5mm expansion card is not really significantly better than the internal audio interface of the FW13, at least in terms of features. I don’t see any indication that it would be arranged differently, as its primary motivation is to have a 3.5mm port on the FW16.
It is a bit disappointing in this regard, and there would be room for a more featureful audio expansion card, but the one Framework offers isn’t.