If you read that thread properly, you will see that
- it’s from 2008 and is for a very different version of Moodle, and
- where people had tried changes, they describe them as 'clunky' eg. some error messages would still pop up about not being allowed to do things.
The simple fact is that guests (ie. non-logged in users) are not able to post in forums even though the permissions suggest otherwise.
There is documentation here about how you can allow guests to post - but they still need to log in with a shared account.