I can see how the two topics are related, but the feature in that thread doesn't really address Calibre's css handling.
Anyway the css manipulation is integral to the conversion pipeline, as the css needs to be flattened and merged to support various conversion features. An ePub to ePub conversion which doesn't manipulate css would require a completely different conversion framework, which doesn't exist.
This is part of why the
Modify ePub plugin is being developed, to cherry pick various pieces of the conversion pipeline and allow ePub manipulation without flattening the css.