I disagree with this (sort of) for a couple of reasons:

1. For one, I think it's hard enough just to keep technical documentation up to date. Keeping separate business process docs up to date with code feels like it would be a collosal task.

2. That said, most software at least starts with some amount of requirements (whether that be a formal spec or a bunch of Jira epics). I have rarely seen any engineer start first by reading all of the old business requirements before trying to understand the code.

3. Even if the business requirements were documented perfectly, I rarely find that they are the major issue with understanding a code base.