Hi xorrkaz,
Apologies if I wasn't clear in my response. Because a TKB Article is treated as a "MessageDiscussion" by the REST API, you can use the "reply" method for that class to post a reply/comment:
/restapi/vc/threads/id/45/reply
Here are the parameters currently available:
message.author (optional) : User - the message author. If this parameter is specified, the caller must have permission to post as this user or the call will fail. The author can be specified as a relative REST API path, such as id/43 to specify the author by id or login/username to specify the author by username, or by an absolute path beginning with a forward slash, like /users/id/43.
message.subject (optional) : string - the message subject
message.teaser (optional) : string - the message teaser
message.body (optional) : string - the message body
tag.add (optional) : string - the tags to be added to the message
label.labels (optional) : string - The labels for the message
message.is_draft (optional) : boolean - true if the message is a draft
message.added_contributor_logins (optional) : string - the contributors to be added to the message
message.post_date (optional) : date_time - the contributors to be added to the message
qanda.is_answer (optional) : boolean - whether or not this is a qanda answer
Not all of these will necessarily apply to TKB article comment (ie. qanda.is_answer), but you'll probably at least want to include message.author (if you're not authenticated as the comment author), message.subject, and message.body. For example, if you're already authenticated as who you want to post with, you can do something like:
/restapi/vc/threads/id/3007/reply?message.subject=my%20comment&message.body=this%20is%20my%20comment