I managed to do a fix for the urn (xsi element from xml files in Magento2) and this is important to forward.
As seen in the upper blog post the problem of unrecognised urn by PHPSTORM
So, accordingly to the latest updated as mentioned “a new CLI command ‘dev:urn-catalog:generate’. (Okay, you can see Eugene pointed it out in comments below!) You should probably save your current .idea/misc.xml file, but I just ran the following command from my project root directory”
Fix urn unrecognised by PhpStorm
bin/magento dev:urn-catalog:generate .idea/misc.xml
Now if you look inside your project folder in .idea/misc.xml – it should be populated, if not check permissions.