Metapackages allow you to group an extension that consists of multiple packages into a cohesive unit.
This works exactly as described in standard composer.json documentation.
If you have an extension that uses more than one package you must use a metapackage as the root package. Otherwise you should not use metapackage.
A metapackage that you submit to Magento Marketplace should be a .zip file containing only the metapackage composer.json file.
It appears that the issue is with the type field in the composer file.
According to the web page provided, the type is listed as magento2-module instead of metapackage as stated in the documentation.
The reason for autoload error is due to incorrect labeling in the type field.