Yeah, this is confusing, the documentation you shared is for data stream lifecycle, it is not the same thing as ILM.
If you have ILM, the retention is managed by the ILM policy, the Data Retention column you see in Kibana only shows information if the retention is managed by a Data Stream lifecycle, so if you are using ILM it will shows up as Disabled.
I'm not sure exactly what is the main difference as I found it confusing and just stuck with ILM, it seems that they do basically the same thing, but in some different way.