That repository is informative. Looking at the unit test folder https://github.com/hmrc/off-payroll-decision/tree/master/test/uk/gov/hmrc/decisionservice/ruleengine it's clear that the software implements a rules engine. What the decision actually *is* is driven by a set of rules files.
Those rules files are found in e.g. /conf/tables/1.5.0-final, which in turn are a translation of the Excel spreadsheets found at /docs. Looks like the civil servants update the Excel sheets, the developers transcribe them to CSV. I've done that before because Excel isn't particularly automation-friendly and end-users have a tendency to break the sheet format, breaking the importer.
Now, there should have been some form of end-to-end testing carried out to ensure that the software does correctly implement the rules as indicated, and the results of that testing should have been documented. They may have been overconfident that the rules engine actually implements the rules correctly. None of this ensures that the rules themselves are correct - that's down to whoever's doing the spreadsheets.