I wrote the case C16156676 after that I discovered that .sdcc files are not auto-associable and I found the article CS190813 that talked about the opposite, the customer asked a way to not auto-associate.
The response of my case is:
In Windchill help center page for 10.0 and same exists also for 10.1 saying that associate and auto-associate is not possible for several object types including Containers:
Associate and auto-associate actions are supported only for CAD documents of type: part, assembly, and drawing. These actions are not supported for other Creo Elements/Direct Modeling CAD documents, such as container, workplane, workplane set, and instance, which cannot form owner associations with parts.
For some reason, this note was removed from later Help Center versions. But as it seems, it was already not supported in 10.0 and 10.1 and is not working in later releases.
PTC Product Development team informed that this behavior should be expected also in current release that there is no support for Container parts (.sdcc parts) to have auto-associate functionality, even it is not supported in WGM 12.1
Please note that we raised SPR 13391369 against documentation.
The document you referred us to is obsolete and has been removed in the meanwhile from the database.