The Shidduch Crisis, it is claimed, affects girls more than boys – i.e. there are many more frum unmarried girls than there are frum unmarried boys
Therein lies your mistake, right in the first sentence. There aren’t more frum unmarried girls than frum unmarried boys. This is not what R’ Rechnitz et al are claiming.
We are claiming that there are more frum unmarried girls
looking to get married than frum unmarried boys looking to get married.
What R’ Rechnitz is trying to do (among other things) is try to even the number of frum unmarried boys looking to get married with the number of frum unmarried girls looking to get married. In your example, instead of 20 frum unmarried boys looking to get married and 40 frum unmarried girls looking to get married left over after 100 chasunos, there would be 20 of each (or 40 of each if you wish) so that they can marry each other in the coming years.