Haven't done much research, just a thought.
However when the DS was released they also said it wasn't a replacement for the GBA, when that was also pretty obviously untrue. Additionally, word is that Nintendo has merged their 3DS and Wii U game development efforts to focus on the Switch.
They also don't really have any 3DS games on the radar for further out than the end of the year. It seems safe to assume that Nintendo fully intends to focus on the Switch going forward.
Did Nintendo really say that? I don't see how they could make that argument when the DS literally had a GBA cartridge slot.
The Switch not replacing the 3DS is at least reasonable in theory (not sure about in practice).
Nintendo had just released the frontlit Advance SP a year and half before the DS and had new GBA models in the queue (the micro and the backlit Advance SP) when the DS came out. And behind the scenes they apparently had a single-screen GBA successor in the works too. So they were fully prepared to drop the DS if it failed.
It sounds strange in a historical context but at the movement it really was just an experiment for them. GBA compatibility was relatively cheap so they bolted it on.