The default is supposed to automatically grant permission to everybody. If this is not happening, then there may be a bug. However, I am unable to reproduce that. When I create a new feed update widget, everyone has permission as expected.
I found a different problem with the feed updates widget. If you point the widget at a different area using "Show feeds from this area", and you choose the wiki index, it never finds any feed entries to show. This is because the wrong ID is used to lookup entries with this selection. It sounds like that's not what you experienced though, because you suggested that setting permissions resolved the issue for you.
A possibility is that another add-on is also overriding XF\Entity\Widget::_postSave and doing so in a way that blocks VaultWiki's code here that rebuilds widget permissions automatically.