I think it extremely unlikely that they would have done anything of the sort. I think the mathematical models they are using are slightly more sophisticated than you give them credit for. (For example, you seem to be implying that they don't know about or understand the issue of false positive test results. They most surely do.) Forgive me, but that's just a silly claim on your part. Presumably, it was intended as a joke.
As to the letter with its Appendix, I don't find it to be particularly controversial, and I don't see it as being particularly inconsistent with my views. It's certainly in marked contrast with the 'open letter' you referenced.
However, the letter does say "... the current debate appears unhelpfully polarised around views that COVID is extremely deadly to all ...". I certainly haven't heard anyone claim that "COVID is extremely deadly to all".