Adreno started by using the fixed function blocks from the Imageon acquisition from AMD but combined them with the programmable blocks of the Qualcomm Qshader GPU architecture.
I was part of the GPU handheld IP group within AMD that was acquired by Qualcomm and remained in Qualcomm for several years after the acquisition.
You are correct about the Adreno 200 being a trimmed down version of the Xenos GPU from the XBox 360. That shipped before the acquisition by Qualcomm.
However, I can also confirm that the GP is right: Adreno 300, which is the first product that we shipped after the acquisition, combined the fixed-function blocks from AMD with the programmable shader processor (SP) from Qualcomm. That SP had a prececessor (QShader) which had never shipped commercially.
Overall, I think it is inaccurate to say that Adreno is basically AMD's IP. It was the combination of both teams, plus the people who had been previously acquired from Bitboys. This amalgamation of people led to a ton of internal conflict, eventually leading to the closure of the offices staffed by Bitboys and the departure of a bunch of other senior people.
Oh man, the eDRAM on X360 makes so much more sense now. I always wondered why that was the case but I wasn't as close to the graphics side of things when I was working in gamedev.
GPUs are always something where people have... strong opinions about. That said having spent a bunch of time with the Adrenos they were pretty solid embedded GPUs and watching the generational leaps from 100/200/300/400 was a ton of fun. The GPU group over there at QC had some sharp people and were always great to work with(at least from my exposure).