"Unfortunately, there is no such thing as a yellow LED"

I manufacture 570-580nm diodes.

"This is because the last part of photosynthesis, which takes CO2 from the air and converts it to sugars will also take in Oxygen and use it for photorespiration."

Atmospheric CO2 will not have an effect on the O2 concentration in the soil or medium - roots import oxygen, leaves export oxygen. They were able to reduce the light required with the CO2 increase due to the current effectiveness of LEDs, and the fact you don't have yellow or green light inhibiting other processes - you remove one limitation only to hit another one - they ran into the CO2 barrier and had to supplement.

Roughly 20 photons = 1 molecule of sugar. They didn't have enough CO2 for the consumption/sugar production part of the Krebs cycle.