The file at https://raw.githubusercontent.com/limetech/unRAIDServer/master/unRAIDServer.plg is 6.2.1. I'm not sure if in 6.2.1 they changed the location where it downloads the file, I didn't see 6.2.2 released on github.com.
After updating to 6.2.1, I see that they did change the location. Now it is grabbing the plugin from https://raw.github.com/limetech/webgui/6.2/plugins/unRAIDServer/unRAIDServer.plg
So in theory, someone going from < 6.2.1 could download that plg file and drop it in /tmp/plugins and then update without having to go through multiple steps