These commands (1) sudo nc -vvv feed.community.greenbone.net 1 873 and (2) rsync rsync://feed.community.greenbone.net are not syncing the feed, they are only initializing a connection to the rsync feed.
As far as I understand, the correct method for updating the feeds depends on which version you are running. You can read this post for background
I think you need to run all the commands as the _gvm user (or gvm user for some installations) except maybe the new Python script can be run as sudo?
Feel free to disclose your installation and version for more direct help.