Kubeflow is an OSS framework for doing distributed data science / machine learning (including Tensorflow hence the name) on Kubernetes.
With the AAD integration it's just much more convenient to give a team of data scientists access to a Kubeflow Kubernetes cluster - AAD with the Kubeflow OpenIDConnect component will handle AuthN & AuthZ. Kubeflow itself sets up different Kubernetes namespaces for each authorized user upon first login.
You definitely don't need to run Kubernetes on AKS or Azure in general to take advantage of the AAD features.
It's nice to be able to get your email (Exchange), identity (Active Directory), productivity/office apps (Office), and underlying infrastructure (Azure), and even business intelligence/analytics (Dynamics) all from one place.
Not sure why using Office would be a reason to prefer Azure over something else. Can you elaborate? Are there some useful integrations one can benefit from?
Regarding Amazon buying Slack. My guess would be it's too expensive. If you look at Amazon's acquisitions history, you would see they almost never spend more than $1B. The one exception I recall is Whole Foods which they paid ~$13B for.