Are you sure about that anecdote around the GBA? It would make more sense that they used the Sharp SM83 core - which is pretty far from a Z80 considering it's missing IX, IY and the entire alternate register set. It certainly does have some Z80 instructions and even some SM83 specific instructions. On the dial between 8080 and Z80 it feels closer to 8080 to me.
Wikipedia[0] does have it as the SM83
[0] https://en.wikipedia.org/wiki/Game_Boy_Advance - "Manufactured by the Sharp Corporation, the SoC contains two processors: the ARM7TDMI running at a clock rate of 16.776 megahertz (MHz) for GBA games, and the Sharp SM83 running at 4.194 MHz or 8.389 MHz for backward compatibility with Game Boy and Game Boy Color games."