It just means that software engineering is difficult even with many testers.

Fixing some of the bugs you find internally doesn't mean you need to fix all of the bugs you find internally.

How about the part where you don't pay licensing to other companies? In other words, if you're developing the software anyway, may as well use it.

Sure. My first instinct is to say that's a small concern, but they do have 100k employees.

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.