I was recently reading about the Ork/Seattle Underground and came across an article about the location on Shadowrun's "Fandom" Wiki. In the article a company called "Fungitek" was mentioned, which sparked my interest for reasons unrelated to this question, and as such I tried searching for it online, and also looked for any mentions of it in the Shadowrun 5th Edition Core Rulebook but couldn't find any mentions of it. As for my search online all I could find were some German wiki pages about it, which leads me to my question: Is Fungitek mentioned in the official Shadowrun lore, and if so, where?
Yes, Fungitek is canonical
Fungitek is a canonical corporate entity in Shadowrun, albeit not a particularly significant one; it is mentioned a handful of times in official SR sourcebooks.
Most significantly it is described in the SR4 book Runner Havens, in a couple of paragraphs about the corporation on page 118:
Fungitek (Ork Underground)
Fungi may not be the most exciting scientific field one can think of, so I was more than a bit surprised when I heard about this company. Fungitek has a few farms in Snohomish, a mushroom beer in the works at its labs, and that seems to be it ... at least on the surface.
It's also mentioned on page 43 of the SR4 sourcebook Seattle 2072, in one of the in-universe editorial asides common in SR sourcebooks:
The Underground is more than just a tourist trap and metahuman shopping mecca, it’s a thriving community in its own right and in recent years has been developing other aspects of its economy. Take Fungitek, for example, a corporate start-up in Snohomish specializing in mycocultures. They struck a deal with the Underground to set up a fungus farm down there for some of their new experimental strains, not only bringing more credit into the community, but also a potentially valuable new food source.
It's also referenced as a subsidiary of the Genesis Consortium in the not-not-official Shadows of Latin America (a planned SR3 sourcebook that was never officially published and later fan-compiled from drafts released by the contributors).
It may have been mentioned in other resources also, but these were the ones I could easily track down by following the index/references mentioned in your linked article and the shadowhelix.de Fungitek article.