When you need to start making money, you suddenly need to a lot more engineers. Now you need developers to make sales tools, compliance, moderation, finance, A/B test engineers for growth ideas, niche features for big customers, engineers to bring down cloud costs, etc.
It doesn't make their engineering less impressive but I think we need to understand the full picture.
At the end of the day, WhatsApp needed to make money to keep their service going.