The site itself was set up (by me) just to host this open letter, just as the site masks4all.co was set up to host the similar open letter that happened for masks in the spring. The credibility isn't of the site itself but of the signatories of the site, which are listed plainly (with any conflicts of interest). Look through the list yourself to judge their credibility.

Thanks for the explanation, I'll look into the sources which you cited so the credibility was very apparent, just not the domain which you've cleared up for me.