The fermentation of tea that is mentioned in this article is not the same as the fermentation that happens to grapes, dates etc in order to make them intoxicants. Rather what is meant by fermentation here is oxidization, i.e., exposing the tea to oxygen for three hours, according to the experts.  

With green tea, the leaves are treated with steam after picking, then they are dried immediately, and not exposed to any oxidization, rather they are preserved with the same properties as are found in fresh tea. 

But in the case of black tea, the leaves are separated after collection and are sorted into fine layers on a net made of wire or sackcloth in order to get rid of excess water, then the leaves are crumbled and sifted, then subjected to an oxidization process, by exposing the fresh leaves to oxygen for three hours, until the tea loses its green colour and becomes dark coloured, after which it becomes black tea.  

If this oxidization is done only partially, we get tea that is partially fermented, between green and black. This is called tannin and it combines the qualities of both black and green tea. 

See: Mawsoo’ah al-A’shaab al-Tibbiyyah by the pharmacist Dr Ahmad Muhammad ‘Awf; Qaamoos al-Ghidha’ wa’l-Tadaawi bi’l-Nabaat by Ahmad Qudaamah, published by Daar al-Nafaa’is; al-Ghidha’ la al-Dawa’ by Dr. Sabri al-Qabbaani, published by Dar al-‘Ilm li’l-Malaayeen. All of these books are arranged in alphabetical order. 

This is what we have been able to find out about expert opinions on this matter. Thus it is clear that the process of fermenting tealeaves is not haraam, rather fermenting them means exposing them to oxygen, and we do no know of any scholar who says that this is haraam. 

And Allaah knows best.