Andrea,
The paper you linked is a review paper, not a study measuring B12 in kombucha. They cite the claim that kombucha has B12, but their citations are not included in their list of references. I’m not aware of a study showing that kombucha has vitamin B12 activity in humans, which is what’s needed before someone would rely on it for vitamin B12. Please see my article, Vitamin B12 in Plant Foods, for an explanation of what’s required before a plant food should be considered a reliable source of B12.

There are publications showing the high levels of b12 in Kombucha and drinking this and/or water kefir allows for ample b12 for vegans. It’s very sad that there is so much misinformation surrounding b12. It is a product of the fermentation process! Below one Link but there are many available.

https://www.sciencedirect.com/science/article/pii/S1674638422001289?ref=pdf_download&fr=RR-2&rr=96751bc55ee88dc1#coi005

I haven’t seen research looking at whether kombucha has vitamin B12, and without it, I have to presume it doesn’t.