> Solein® is made from natural single-cell organisms, which are grown in a fermentation process. Water is split from the air with renewable electricity into hydrogen and oxygen. The cells are fed CO2 from the air, hydrogen and mineral nutrients.

There is no nitrogen mentioned in this explanation. You cannot make amino acids or proteins without nitrogen. These organism either have to be able to do nitrogen-fixation from the air, which is a really difficult and energy-intensive process or the nitrogen has to come from some growth medium. It certainly doesn't make a good first impression that they "forgot" such an important part in this description.

In general the interesting part of such a scheme is how much energy it uses. Converting CO2 to organic molecules and nitrogen-fixation are very energy-intensive processes, the problem isn't that we can't do this but that the amount of energy is incredibly expensive compared to using photosynthesis in plants.