The thrust of R’ Moshe’s teshuvos in the Igros point to the heter not being recommended unless it’s sha’as had’chak, the teshuva to R’ Weinfeld merely enforces it, and there could be any number of reasons it wasn’t printed in Igros, but l’maaseh, he wrote it.
That the heter should only be used b’sha’as had’chak, and that it’s muttar meikar hadin, are not mutually exclusive.