I am not any person named in the linked page, though I have met some or all of them. I am not affiliated with MIRI in any way. I did not post the linked page and I do not know who did.
The linked page is obvious slander. But its creation is a serious matter; the author is threatening to manufacture evidence. Thus, it should be handled the same way as a death threat: with an investigation to determine who sent it. The site is hosted on EasyWeb; the domain name admin contact details point to a proxy called myprivacy.net, but the author is not very technically sophisticated (the page was authored in MS Word) so an appropriate subpoena might suffice to identify them.
Also, Mendes & Mount might want to make a public statement as to whether or not they represent MIRI. The page mentions them by name, but they're located in the wrong state (New York) and none of the practice areas listed on their web page are relevant.
I understand fabricating evidence is bad. I understand that libel is bad. But I don't see how threatening to libel is that bad (on top of the actual libel).