There is no contradiction between Rav Miller’s strong suggestion that girls marry straight out of school (if not before), and the R”Y’s letter. Rav Miller, like other Gedolim, suggest girls marry young. This should be followed.
The R”Y want to help older unmarried girls. So they suggest close in age marriages. And they suggest shadchanim provide more help for older girls. Neither suggestion conflicts or changes what Rav Miller and other Gedolim have long advocated.
The only conflict, perhaps, is a few other folks (sometimes falsely wrapping their ideas in what the R”Y said) saying that 17 or 18 year old girls should not get married at that age, or that no one should help 17 or 18 year old girls get married. The R”Y not only never advocated as such in their letter, it goes against the aforementioned words of our Gedolim that has long been advocated.