Realm is a very different product than Firebase. At its heart Realm is based around a first-class native database, with support for complex data models, advanced queries, data binding, multi-threading, true offline capabilities and much more, which is only weakly, if at all, supported by a BaaS like Firebase.
That does not mean that Firebase is not a very useful and capable product. We are seeing tons of users who are using Firebase in combination with Realm, since Firebase also offers features like ads and analytics. So often the combination give a better coverage of all the needs of an mobile app.
Google is a big company that gives its individual teams a lot of leniency in choosing the best technology for the job. The fact that some of the teams have chosen Realm, just means that they judged it to be the best solution for their specific problems, and is not an endorsement or rejection of any specific technology.