Selecting journal reviewers through cover letter
They don't require you to send a cover letter to five referees, but just to indicate in your cover letter the names and contact info of 5 potential reviewers. It's just to help them finding reviewers for your paper (as they say, they might not use those you provided). But I don't think you need to contact the reviewers first. 

As far as I know, it's a pretty common procedure, I've seen it for several other journals.