What you’re saying is absolutely true for the mapping scheme commonly known as LoROM.
The SNES memory map can seem very confusing. What made it finally “click” with me was the realization that the SNES asserts the /ROMSEL pin on the cartridge connector for these address ranges:
Code:
00-3F:8000-FFFF
40-7D:0000-FFFF
80-BF:8000-FFFF
C0-FF:0000-FFFF
In essence the SNES expects the cartridge to put something on the bus for these address ranges, while internal MMIO and RAM may have priority in other ranges. The peculiarities of all mapping schemes made a lot more sense then.
That said, any ranges not actually used by system MMIO or RAM are also usable on external buses, but I don’t think any official cartridges or peripherals do that.