You are quite right, KNX is very autonomous. All home automation busses typically are, be they KNX, Z-Wave, X-10, PLCBUS, whatever.
This is for a lot of reasons, but really, all that is relevant here is that LinuxMCE merely acts as a sort of submissive slave. It will politely ask, through its computer interface to the bus to do certain things, "Turn on the bedroom light", "Set the air conditioner to 70 degrees F", and assuming things are kosher, the devices on the other end of the bus will comply, and do them. Similarly, if an event comes across the wire, say the thermostat says, "The room temperature is now 70 degrees." LinuxMCE can take this event and react to it somehow, however you wish.
KNX can take this to an extreme degree, and it's a good thing. It means, that if the computer goes boom, the sensors (in this case, the switch on the wall) you've mapped to the actors controlling the light, will still function.
All that LinuxMCE cares about, is its interface to the bus, and that it knows what is on the bus, so it can know where things are, what commands can be sent, and if an event comes across the wire, where it came from.
So yes, you don't NEED the computer, but the computer opens up additional possibilities, and as long as there is an interface to the bus, and LinuxMCE knows how to talk to it, LinuxMCE can be used to either send commands to it, or listen to events from it.
It also opens up the possibility to mix and match. If you have, say, Z-Wave, and KNX devices, you could attach computer interfaces to the LinuxMCE core (or media directors, it really doesn't matter, especially if the media directors stay on), and then issue commands from LinuxMCE and LinuxMCE will do the right thing. It makes LinuxMCE a sort of United Nations in the middle, and ultimately allows you to have a wider selection of choice.