Part of the story that is mentioned but not discussed is that X_10 had a fixed and tight schedule. Under that constraint you expect some compromises for writing

Part of the story that is mentioned but not discussed is that X_10 had a fixed and tight schedule. Under that constraint you expect some compromises for writing the mental model down in a way others can learn, because that model presumably lived in her head. It might have been possible to say, great job you delivered on time, now you have as much time as you need to write your mental model down. On full pay of course, and we won't count that against you as non-technical work in the next performance review. That increases the upfront cost of SVC but is an investment that pays back interest as soon as someone else has to fix anything.