That is a new one. I've never seen that before. Casino sites are usually pretty thin content wise so they often try to publish something on them, usually poorly written articles on any casino related topic. As you say its for purposes of associating with keywords in the search engines. Maybe they could also be trying to reinforce a good review by telling their visitors how wonderful the reviewing site is, and the reverse of course, attempting to undermine trust in a site that gives them a bad review.
It seems a questionable idea to me, given there's a chance that someone is going to read about these wonderful portals (assuming the reviews are positive) that has hundreds of casinos on it and go looking for it instead of playing at their site. It's a similar situation to casinos that recommend affiliate sites as mediators in disputes in lieu of an actual independent mediator, such a dumb idea and a turn off as an affiliate that they are actively pointing players to other sites from which they are unlikely to return.
What is the casino?