I assume you mean
THIS article?
Although it is informative article for basic understandings, I would
never use the setup (instruments connection) as shown on his first picture. I've checked article twice to make sure if he mentioned somewhere why it shouldn't be done that way, but he didn't. That is, I would check elsewhere before making buying decision.
Let me elaborate...
If you plan to record instruments, or instrument+microphone(s), you should never mix those before sending to DAW. Signal of each instrument must be sent (recorded) on DAW separately. Mixing is a final step after recording is done -that's the purpose of DAW. If already mixed instruments are recorded (i.e. keyboard & guitar), then it's impossible to adjust/balance levels in DAW later.
And second, keyboard is a stereo (two audio channels) instrument and should be recorded that way. That is, keyboard & guitar need three channels (and if there's a singer, additional channel is needed).
Conclusion...
Connection shown in that article is good enough solution for recording live performance, or if we are extremely budget limited. But for home/studio recordings (say, a band wish to present itself to someone), it's totally unacceptable.
Am I nitpicking about this? Not really. Look at the picture again... for what it's accomplished on that picture, mixer actually isn't needed at all.
Just my opinion,
Bogdan