Also I can employ a $75k - $100k/year engineer and say follow this protocol for testing. If I want someone who can optimize test case 100% of the time with zero mistakes ever then I might have to pay $150k - $200k / year.
Anyone who demanded the real hashing algorithm in this case doesn’t understand how hash collisions work and what problems you need to solve when using hashing. Your “100% accurate test case” serves very little value.
It’s the same as requiring a real person perform the checkout workflow in your shopping cart integration tests. It’s “100% accurate” but it’s a massive waste of resources and provides terrible coverage of the many other edge cases.
If we wanted to change how the hash is calculated all extant instances of the system need to dump to backups, then be painstakingly restored after updating. Possible, but certainly not something you'd be doing for giggles.