I don’t think you can post ‘anonymously’ in the sense that there is no account related to your post, you’ll always have to create an account, but you can of course use a one-off username and even email address if you want to. However, you can delete your account and then apparently all ‘your’ posts appear as “[anonymous]” whether you intended this or not. (And in this case, it seems the original poster just created this post with their usual, non-anonymous account, but then deleted their account.)
I don’t think you can post ‘anonymously’ in the sense that there is no account related to your post, you’ll always have to create an account, but you can of course use a one-off username and even email address if you want to. However, you can delete your account and then apparently all ‘your’ posts appear as “[anonymous]” whether you intended this or not. (And in this case, it seems the original poster just created this post with their usual, non-anonymous account, but then deleted their account.)