So a recent thing indeed. Living in São Paulo I never heard of jambu (I also do not drink alcohol, and apparently it is more interesting to consume with alcohol, so it might explain).
I've had jambu in São Paulo many times, although I admit that I visit the city multiple times a year just to eat. It's easy to find tacacá, pirarucu, chicken/duck with tucupi and so on.
I would imagine so. There is a lot of good things in SP, probably never heard of a lot of them.
São Paulo resident here as well... You may find it in several restaurants around Paulista avenue.