> I happened to ask an ex-WhatsApp PM about this. He told me that, especially in small groups, when people communicate with A, B, C, and C leaves quietly, it could cause misunderstanding
I don't dispute that your project manager honestly believed that to be the reason, but if my corporate experience is anything to go by, once this got run past sales, legal and marketing, the reason will have morphed into “if we browbeat people into not leaving groups, that means we can increase reach for our advertisers” Note that it's beside the point whether WhatsApp currently post ads in groups or not (no idea if they do, I don't use it). The idea will have certainly been considered.
Biting the hand that feeds IT
