I have no clue the intent of this buy, but Google Cloud does have an IAM feature today[0]. I'd bet they are looking to pull in talent to help improve that feature, or maybe to make the IAM more than just IAM and handle all the things that Bitium does today.
You have to remember that Google Cloud also has bought StackDriver and Firebase and continues to grow those products.
(disclaimer: Google employee that wishes he worked on cloud)