Correct. It's easy to be confused- They're technically both digital, but easier to think as Audio being sound, and Midi simply being information about that sound. Midi makes no sound on it's own. You can record both in Logic. Much like words or picture to the printed page- once Audio is committed it's mostly finite*, whereas with Midi you can continue to edit your performance/recording.
In the Ye old days we could only record Midi into what were called Sequencers (now called DAWs). The Sequencer (hardware or program) would record Midi information- this was way before you could record audio- and you'd have to have a physical keyboard or module to produce the sound and a Midi Cable. Many members here know the history better than me, but Logic actually started it's early days as a sequencer program from Emagic - and it ran on PC too.
Like many other modern keyboards controllers, the USB cable serves as Midi cable- not Audio. You can commit the audio from your Nord (the 1/4inch jacks) to an audio track in Logic. You're then hearing the Audio from Logic (and the Duet), not from the Nord After recording said audio track, If you disconnect the Nord (or simply turn down the volume) you'll still hear the audio track. If you send Midi to the Nord, (via that cable) you'll be hearing audio from the Nord. If you send or play Midi to a Logic internal synth- you'd also hear from the Duet. Hope that helps! David's book does a much better job of explaining, and highly recommended.