It is kind of interesting seeing the fear developers have over "everything in finance is on excel." It kind of reeks of armchairism where one assumes the worst immediately and assumes this assumption has somehow not been realized by the domain experts in this space who are in fact also smart spend all their time on the thing.

To an accountant, excel spreadsheet is a source of truth. There is no undetermined behavior. They can look at the calculations underlying the spreadsheet and understand what is happening, no different than a developer looking at source code. They do in fact have their own forms of unit tests considering these data are audited in a far more rigorous fashion than most any code that ships with unit tests.