You didn't read their letters carefully. Nowhere do they write that outside minyanim themselves are dangerous. That would be silly. I spoke to them and also saw what the Rabbi of YIW wrote clearly. It was a gezaira maybe people might go inside or not keep rules.
The shtikel you posted did not have any solid rayos at all. In fact we always find שלוחי מצוה אינם ניזקין and שומר מצוה לא ידע דבר רע when it come to kiyum hamitzvos. To say differently is a big chidush.
Seems to say so pretty clearly:
"Many individuals have been asking Rabbanim to demonstrate leadership by issuing clear guidelines for the establishment of safe outdoor Minyanim. We all understand that Tefilla Bi'Tzibbur is a crucial imperative for Klal Yisroel, especially in a time of crisis, and that davening together could provide us the זכויות and the connection that we so desperately need. People are requesting that Rabbanim, in conjunction with medical professionals, create guidelines for safe and legal Minyanim in our neighborhoods, with all participants wearing masks at safe distances, perhaps on separate properties, with safeguards in place to ensure that there is no significant risk.
We, the undersigned Rabbanim and Roshei Yeshivos, after much thought and careful consideration, feel obligated to communicate- in unequivocal terms- our united decision to reinforce our previous mandate, and bid our community against the creation of any Minyanim, as well as to cease unauthorized Minyanim. This includes porches, front-/backyard, Minyanim--with no exceptions whatsoever. We will continue to reassess and reanalyze this decision on a regular and ongoing basis as new developments unfold.
Pikuach Nefesh is the greatest imperative in the Torah, and overrides almost any other Mitzva. Although on an individual level it may be possible to safeguard against risk, it is impossible to allow the establishment of Minyanim across a community without also accepting the unavoidable violations of safety guidelines and inadvertent health risks that will inevitably result (especially with hundreds davening three times a day, every day). כל המקיים נפש אחת מישראל...כאילו קיים עולם מלא-- סנהדרין לז. Easing the prohibition would be, at the very minimum, a Sa'fek Pi'kuach Nefesh that could endanger participants, and could increase the likelihood of illness and loss of life in our community, chalila vi'chas. And this simply cannot be allowed. At this point in time, this overarching principle is docheh the imperatives of Tefilla B'tzibbur, Aveilim reciting Kaddish, and many other Halachos. We call upon each and every member of our community to adhere to the safeguards that are essential for the protection of us all. Let us do our absolute utmost in following the Torah's mandate of ונשמרתם מאד לנפשותיכם."