I recently started brewing Kombucha and one thing I noted is how they tell you it's required to do open vessel fermentation. The main reason why I am asking is because I wanted to ferment it under pressure in a keg, but based on what I can read - that's not advised as it can kill the scoby. So what's the difference why beer can be fermented under pressure and Kombucha can't?
Because the fermentation of kombucha proceeds in two steps.
In the first step, the sugar in the tea is fermented into alcohol. Yeast can do this with or without oxygen. This is done by yeast, a single-cel fungus.
In the second step, alcohol is fermented into mostly acetic acid (vinegar) by bacteria. The bacteria need oxygen for this process. So, if you would starve them from oxygen, they would not be able to feed on the alcohol, and would die. Another effect would be that you would not get kombucha, only fermented tea.
It is not the pressure that would kill the scoby, but the fact that the bacteria would not get oxygen that would kill the scoby.
(One brainfart: if you have the material to ferment under pressure, you could of course check if you can apply air under pressure (not pure oxygen) instead of CO2, so that the bacteria can still turn the alcohol into vinegar and you get a nice carbonation in your kombucha?)