Paper rejected based on one very short wrong comment

1. Wait. The editor has to assess if your claim is correct. People are busy. In addition, they may need to ask a more expert colleague for a quick opinion; this is not a full round of review, but still it will take several days. I would give them at least two weeks, personally (but publication times are heavily discipline-dependent).
2. If the editor agrees with the reviewer, consider seriously the possibility that they are correct and you are wrong. Ask privately for another opinion to a collaborator/colleague you are in good terms with, just to understand where the truth lies.
3. If you conclude that the referee is completely bonkers and their claim is ridiculously wrong, ignore their remark, submit somewhere else, and make a note for the future that this is not a good journal.
4. If you conclude that the referee is wrong, but their one is a reasonable mis-interpretation and someone else may get it wrong, too, then revise your introduction: mention mr. X's papers and explain in your manuscript why they don't solve your problem. Then, submit somewhere else.

(Note that case 4 is much more common than case 3.)

You know what the referee said in the part of the review meant to be seen by the authors, and you know that the paper was rejected. You don't have necessarily have access to all the communication between the editor and the referee.

In particular, there are often check boxes for the referee indicating how she or he ranks the importance of the topic of the paper, the relevance of the topic with respect to the journal and the readership, and the priority for publication.

You also don't necessarily know what's already in the hopper for this journal. They might have a backlog of extremely high quality papers, and are only accepting the best of the best. There are certainly enough papers that receive good reviews that don't get published -- just like there are tons of good grants that don't get funded.

So, there's a chance that your paper was an excellent paper, and just not a high priority for that particular journal at this particular time.

There's also the possibility that the referee is right about how applicable the work of Mr. X is, and you just don't see it. (No offense meant -- all we know about you are your initials, and that you say you've had a paper rejected). Maybe what the referee is trying to say is that Mr. X's work is solid enough, and even though you have a new approach, the conclusions drawn are similar enough that a revisit isn't high enough priority for publication in that journal. We have no way of knowing. If you feel this isn't on the mark, when you resubmit elsewhere, be sure to include the work of Mr. X in your discussion, and show the reader why your work is different from that work, and why your current efforts are important to consider.

The best advice I can offer is to depersonalize the process. You'll hear a lot of stuff during peer review, and it's real easy to let it get to you. Don't let it get to you. With experience, you'll learn what your referee pool wants to see, and you'll give it to them, and have an easier time pushing your papers through.

The dumb reviewer is a common problem, and fits Occam's razor much better than the dangerous conspiracy that would destroy a prestigious journal's treasured reputation if the truth got out.

The dumb reviewer is especially problematic because not only are they not qualified to provide a helpful review of your paper's technical content, the fact that they got themselves in over their head by accepting it means they are doubly motivated to find a way out. Besides of course actually putting in the (especially hard for them) effort of comprehending the paper. The easiest and most frustrating response one gets from such reviewers is to seize upon a "fatal flaw" which would make the paper completely unsuitable for the journal and not worth trying to fix... or finish reading, fortunately for them.

Sending an email to the editor may make you feel better (I've done it) but won't accomplish anything. You certainly shouldn't rant at them.

What you can do that actually matters is write a better paper. If the dumb reviewer hadn't gone for the easy out, they instead could have demanded you to explain far better what your novel results are and how they fit into prior research. This needs to be something even a dumb reviewer can immediately understand when they read your abstract and intro. If they are guessing, wrongly or not, you didn't do your job well enough.

Two months is nothing, by the way.