There is no reason to doubt the teshuva. He wrote it. Not being in the I”M means nothing. There’s no stirah to baal nefesh yachmir. He wrote that when it was very difficult to get CY. When it became easier, he wrote tzorich l’hasig. The halacha didn’t change, the circumstances did.
Rav Dovid may hold differently than his father about tzorich l’hasig. That doesn’t change what Rav Moshe wrote and allowed to be published. Nor does that teshuva make cholov stam assur, it just says explicitly what Rav Moshe held should be done l’maaseh under easier circumstances. ???”?. It makes sense.