Using WhatsApp as an example of a lean engineering org should almost be banned at this point. WhatsApp had a high performing engineering team that used basically the perfect set of tools to build their application (which also had a narrow feature scope; plaintext messaging). Even with hindsight there is very little you could do to improve on how they executed.

Just because WhatsApp scaled to almost half a billion users with a small engineering team doesn't mean that's the standard, or even achievable, for almost all teams.