Hey Jason,
We've checked your site and found that while retrieving the categories to post discussion its not retrieving the guest user id correctly, so re-saved back in user manager options.
here:
https://monosnap.com/direct/rEPCxL0sTRzibl2HjMBVnOnQNoRsL6
So, now the user group is retrieving correctly and guest users can post discussions.
Please take a look at this and see how it goes.
Update us if the still issue persists.