Just because something is used internally doesn't mean the product manager is going to prioritize fixes for internal bug reports. Testing internally does not magically add more programmers to the project.
Sorry, I was introduced to "dogfooding" in Dreaming in Code. I believe there was even a chapter named after it and it was a pretty big deal about how you dogfood to maintain quality. So when I hear the term that's what I assume is implied.
For most day to day usage issues, dogfooding is a great way to hammer out issues. Will they find issues with every use case? No, but they will find some. Also, generally, the more diverse and large the organization, the more use cases will be dogfooded (assuming the bulk of the org is encouraged/required to do so).
When you are talking a company of 90k+, encouraged dogfooding will hit a lot of different use cases.