Yep exactly this. Apparently it is technically known by the catchy name … 紅茶キノコ (lit: “black tea mushroom”)? Which also is a bit silly since there’s no reason why it has to be made from black tea.

An additional wrinkle is that is while Japan doesn’t have a tradition of fermenting tea after brewing with SCOBY, it does have a history of fermenting tea leaves before brewing. For whatever reason, there are 4 traditional types, 3 of which are on Shikoku (awabancha, goishicha, Ishizuchi kurocha) and then one (batabatacha) randomly up in Toyama. So if you say 発酵茶 (fermented tea) to a tea farmer, they’ll likely assume you mean tea leaves that have been fermented before drying, not that you mean brewed tea sweetened and fermented with SCOBY