Interesting. So in a nut-shell, they're just using a coil of silicone tubing as a CO2 permeable membrane and measuring the CO2 concentration of the air inside the tubing.
It's a neat idea. Cheapest NDIR CO2 meter I could find was still $170, so considering a small air pump, silicone tubing, non-CO2 permeable tubing (e.g. copper / PVC / Stainless), time involved... we're looking at about ~$200 for a CO2 meter capable of 5-min time resolution.
I may just build one of these. Wouldn't take me much time at all. Now if I hadn't already blown my aquarium budget on upgrading my filtration system... dag-nabit!!