From the number of people and the wide range of field and institutions signing this letter, I suspect a very strong CCP secret united action behind it to paint a good picture for CCP reputation. The most probably org that manipulated this letter is the so called "Chinese Students and Scholar Organization" that exists in every country and directed by CCP embassy. Only such an org can enlist by its members's connections so many different field people to sign this letter.
On the other hand, I think most of the signers are sincere in their actions and belief and did not realize they are being manipulated by CCP.
Cut away the part that praise CCP, the other part of the letter is mostly correct I think, countries should take actions before it is too late.