I have no idea what that Q/A is referring to. My best guess is that it's out of date.
Here's what I see in the device tab:
I can configure things like the buffer size and sample rate, but no way to change how many USB channels I can use (except through changing the sample rate). I'll edit this post with results from your program in a moment.
Edit: Tried it out.
Drumfix: Took me a little while to figure out that I needed to send audio in to the interface while your thing was running, but when I did everything appeared to work for several minutes (which is way better than I get with ALSA). However, once I switched tabs to post this I heard the audio drop out and saw that it had switched channels. There wasn't any output that suggested why, just a bunch of "motu control:" lines with no data. I made a quick change and confirmed all those control packets were empty.
Also, when I first started your program and things seemed to work, the channels were still off by a multiple of 8. I think they were 9 & 10 instead of 1 & 2.
One other thing I noticed is even after starting and stopping your program the channel assignments seem to stick. That is, if I stop your program and channels 1 & 2 in are playing over 9 & 10 out, then I stop and start it again, I find 1 & 2 continue to map to 9 & 10.