Getting a light source that always provides the same spectrum is extremely difficult. The engineering alone is difficult and the quality control on the parts and assembly is also expensive. I think if you think about how hard it would be to get a light source that regardless of medium provides accurate color from reflected sources would be very hard. Precise white point is hard, its even harder when you are dealing with large temperature ranges, humidity levels and varying background lighting, source materials and every other natural thing that can get in the way of accurate light levels.
Commercial parts that provide what they are talking about are in the $500 range. Without commercial volumes the price could be 10x that much. Normal consumers don't generally buy these but they are essential in the printing and art reproduction business and doing the hardware right is very hard and expensive.
Great article, as someone who's done enough amateur photography that I was forced to buy monitor color calibration hardware and go through color calibration routines to ensure I got good reproduction of what I was looking at on screen. When I purchased though I wasn't aware that the commercial providers expect you to buy new hardware every few years because they abandon all support and driver updates. I can't afford to buy every couple years which left me having to either keep older hardware and software around to do photography on or to simply stop calibrating. I wasn't aware of the OSS options. Argyll made my older but still perfectly functional hardware usable not only in Linux but newer versions of Windows without being forced into another multi hundred hardware purchase. So thank you. I'll add my voice as someone that would be very interested in a OSS reflected light spectrophotometer that I could use for color calibration. Even if it was more expensive than the commercial options it would be better for no other reason than the hardware and software being OSS would be far less likely to be abandoned.