[@robkk Moderator 11 years, 3 months ago]

It is just the quicktags api from WordPress. You see the same button when entering the html mode when creating a blog post.

See if this post helps, it says tinymce but there is also quicktags code in there.

Add TinyMCE and Quicktags visual editors to WordPress comments form

you can also use the basic comment quicktags plugin