I would say that’s not true 
You can overwrite next build number value in settings tab:
You can set lower value and achieve duplicate build numbers.
However, build number is by definition user friendly autoincrementing integer.
Changing build number is rather something hardly ever done and if it happen it is rather one-time action for given app and it is changed to higher values. I can imagine cases like migrating from another CI or splitting/merging Bitrise apps when it can be useful. I don’t expect that feature like that: Change build number from build script will be implemented soon.
@PicOrgApp if your bigger problem is to make Bitrise builds more easily discoverable you can try to solve it in another ways. Build number is not designed for such use case.