The problem is similar to #2179
#2179 is not actually a problem because of the solution I found and described there.
But when an extension need to import a standard Magento 2 Less file the solution will not help because of the folder structure:
var\view_preprocessed\css\adminhtml\Magento\backend\en_US:
--------css
--------Custom_Extenson
So the Custom_Extenson can not @import a Less file from the css folder (this is the folder with standard Magento 2 less files) as it can not use reverse paths.