I don't think they meant that without utilities and modulation, you wouldn't be enough to make good sounds. The Atlantix can totally function on its own as a synth voice sequenced by the Metropolix and be patched and repatched to do crazy stuff. However, things like
- free-running LFOs and function generators with loopable envelopes that let your sounds get off the grid
- VCAs for controlling levels of not just audio, but also CV to make things extra dynamic
- envelope followers to have very specific experiments in modulation shape
- switches of various kinds to move and route signals in unique ways
- clock dividers to give more varied timing events in your setup
...are very simple circuits in the vein of classic modular, but they will make your one synth voice suddenly feel like a massive environment that is constantly evolving. There are obviously so many other random things I can't get into here because it would take too long, but these little things are where the fun it. Atlantix has the sample and hold, noise, rimg mod, and lots of cool stuff if you want to start here and get comfortable, but that's only the beginning.
I would recommend looking at these modules: Make Noise Maths, Make Noise Function, Divkid Ochd, Doepfer Octal Linear VCA. You don't have to buy them to get going (or at all), and this stuff you have in the picture here would be a good start while you ponder what else you might want, but at least look at these modules as stuff your module doesn't have that might complement it well.
You may want to consider getting the Metropolix Solo, where it has its own case with MIDI in and out. This will free up space in your rack to have other cool stuff without anything being too cramped.
Do you already own any of this?