Initial records used didgeridoo and trumpet interchangeably, but today in Australia, the didgeridoo is still considered a distinct instrument from the bamboo trumpets or similar pipes noted in early colonial writings.
For instance, here is a quote from Sir Baldwin Spencer’s Recordings of Australian Aboriginal Singing: “Also recorded on the 1912 series are accompanying sounds made by the “conch”, Spencer’s name for the Australian wooden trumpet, currently called the “didjeridu”*.”
In The Australian Aboriginal by Herbert Basedow, here is a quote: “The instrument which is capable of producing the loudest, and, at the same time, most weird sound, when correctly manipulated by an aboriginal, is the bamboo trumpet, otherwise known as the drone-pipe or “didjeridoo.””
Early explorers, missionaries, and settlers often described unfamiliar Indigenous instruments using European analogies.
To their ears, the deep resonant sound of the didgeridoo resembled a “trumpet” (or sometimes a “horn” or “pipe”), so they used those terms rather than Aboriginal names.
In addition, “Didgeridoo” is not an Aboriginal word. It appeared in English writing in the early 20th century, likely imitating the sound of the instrument (“did-jerry-doo”).
So when you see old sources call it a “trumpet”, they were trying to compare it to a European brass instrument. When you see “didgeridoo”, it’s the later onomatopoeic English term that spread widely in the 20th century.