The reasoning why as pointed out by the article makes it even more mind-boggling to me. It includes the GBA CPU almost by accident, because Nintendo took a Matroshka doll approach to backwards compatibility.
The Nintendo 3DS contains the entire system architecture of the Nintendo DSi for backwards compatibility (the DSi CPU also functions as a supervisor CPU of the system), which is mostly an overclocked variety of the original Nintendo DS architecture. The first two Nintendo DS models contained a GBA slot for backwards compatibility and therefore also contain the GBA's original ARM7TDMI. The Nintendo DS also used this as an auxiliary processor for various things. The reason the GBA hardware is there is because Nintendo kept shipping their previous gen hardware in the new generation twice over.
As part of the that Ambassador Program you mention, Nintendo released a boot firmware for the 3DS called "AGB_FIRM" to boot into Game Boy Advance mode (another being "TWL_FIRM" to boot into Nintendo DS mode). If you hack your 3DS today it can be made to load other Game Boy Advance software and essentially achieves perfect backwards compatibility and runs GBA games with perfect accuracy, modulo any GBA peripherals that were popular.