Probably because they don't care if it's made by STMicroelectronics or by GigaDevice. Most likely it is made by GigaDevice, but maybe they thought calling it a GD32F103C8T6 would be meaningless to many people and calling it an STM32F103 would be dishonest and so this was a compromise. Or maybe they found a third source that would have an even less recognized name.

How did things work back in the day of (official) second-sourcing? Did you just refer to a MC68000 even if you meant generically any of MC68000/HD68000/MK68000/etc? I guess it helped that it had such a unique model number and any of them could just be referred to as a 68000. I guess calling this a 32F103 would be the equivalent in this case.