MongoDB Atlas M0 (Free Tier) BI capabilities
Your BI Connector links are from different documentation contexts (Atlas vs On-Prem). For an overview of deployment variations, see About the MongoDB Connector for BI.


  Checking the documentation again I realized BI Connector is not supported in M0: https://docs.atlas.mongodb.com/bi-connection/#bi-connection


The MongoDB Atlas documentation is referring to "BI Connector for Atlas", which is an integrated feature allowing you to add a managed instance of BI Connector to an M10+ Atlas deployment. Since BI Connector operations may be CPU and memory intensive, adding a managed BI Connector is only supported for dedicated Atlas clusters.


  This is the documentation I used originally and it does mention limitation for free tier but doesn't say it's not supported at all so I'm confused what's the root cause for this error message and if it should work or not: 
  https://docs.mongodb.com/bi-connector/v2.1/tutorial/connecting-to-atlas/


This is the product documentation for the On-Prem MongoDB Connector for BI. If you install this in your own environment, you can connect to a supported MongoDB deployment (including the Atlas free tier) subject to any caveats for your deployment environment.

For example, the Atlas free tier cannot run aggregation pipelines using the allowDiskUse option. This means aggregation stages will be limited to using 100MB of RAM per stage, so queries might fail if one of the intermediate result sets is too large.

See Atlas Free Tier Limitations for other considerations.


  unable to load MongoDB information: failed to create admin session for loading server cluster information: unable to execute command: server selection error: server selection timeout


This error message indicates inability to connect to your Atlas deployment. Make sure the source IP for your BI Connector instance is whitelisted in Atlas and check the connection string details.