I'm not sure if interfaces are descendants of Object. You can define an interface that specifies a contract for behavior and until there's an implementation of that interface, inheritance mechanisms don't really come into play yet.
It's not a bad thing to want to thoroughly understand what the author is saying but frankly, things like this are not really a practical concern and it kind of makes me wonder whether this is somehow going to end up in a future list of "interview questions" along with a bunch of other annoying and practically irrelevant "trivia" questions about Java.
Understand but don't over-analyze.