Asian cultures called various hot beverages tea 茶 before some Westerner decided that they are wrong. Sure there is green tea from that plant but Asian cultures also had mint or chrysanthemum tea using the same 茶 character (pudina chai in India for mint tea).
If anything, the Westerner who decided that beverages made from only that specific shrub is called tea was the wrong one. Broader uses predate your definition.
Technically, these are all decoctions, and "decoction of tea (the plant)" has become just "tea", which is now colloquially replaced "decoction".
So in the sense I was using tea, as a replacement for "decoction", coffee is a "tea", insofar that the replaced word, "decoction", boiled plant matter drink.
Language isn't quite as black as white as we'd sometimes wish it was.