Because of this: "The purchase was made on the same day after Google announced that it had agreed to sell the Motorola brand to Lenovo for $2.91 billion"
The purchase was made after the announcement. Furthermore, it was part of the Motorola transaction itself, and Google didn't exactly gain anything in the short term (Lenovo shares tanked in the days following the transaction).
Edit: I misread the post, they did buy it after the announcement which means it's public knowledge