Is it ethical to cite a reviewer's papers even if they are rather irrelevant?
Don't speculate about who the reviewer(s) of your manuscript are. Nothing good can come out of having the knowledge. In fact thinking you know who the reviewers are can cause you quite a bit of grief.

Forget about who authored the review and focus on the facts. You think the paper suggested is tangentially related and not really relevant. Therefore you think you shouldn't cite it. As in similar situations, the next most natural thing to do would be to not cite it, giving your reasons. It would take a pretty unscrupulous reviewer to reject your manuscript because it doesn't cite his paper, and even if the reviewer attempts this, he would also have to convince the editor that he's actually rejecting your manuscript for valid reasons. Remember the editor can see who the reviewer is, and will notice if the reviewer is pushing his paper even if the relevance is tangential. Further, remember that the editor can accept a paper even if the reviewer recommends rejection.

Finally, about whether to report this, there's no point. You won't learn anything about it anyway because the editors can neither confirm nor deny the identity of the reviewer.