That’s taking a strictly algorithmic approach, and assumes the rules are equally understood by all parties and have no inconsistencies or undefined states. And as an example, I happen to know TaxCycle is unusually responsive to customer feedback on releases and offers early previews. Here’s a list of improvements from 2017 feedback: https://www.taxcycle.com/News/Your-feedback-becomes-features...
https://www.railinc.com/rportal/documents/18/260641/Guidefor...
The software for it has to work. There will be months of work implementing the rules engine before you have anything to show the customer besides a mock-up..
You won’t really need two week sprints to get feedback from customers. It either implements all of the rules or it doesn’t. I can imagine the same happening for tax software.