Hi Sara,
Thanks for reaching out; I'm happy to help! Annotating comments in Review 360 is not currently possible, but we're tracking requests for such a feature, so I'll make sure our report is updated with your voice. Here's a look at how we prioritize such ideas! We’ll update this discussion if this feature makes it to our feature roadmap.
Regarding your other question on what the article meant by the "best experience," users who are signed in with an Articulate ID in Review 360 won't be able to annotate their comments, but they'll be able to enjoy features such as seeing badges for unread comments and changing their email notification settings.
If you require any further information, please let me know!