I believe the primary difference can be summarized in two words - data analytics. Twitter has a ton more analytics options for its users than WhatsApp appears to have mostly because, like Google, Twitter is more of an advertising platform than a utility platform as far as its actual business model goes. WhatsApp is potentially b2c as a business while I'd argue Twitter is really b2b. My experience with enterprise businesses makes it hard for me to believe that going b2b is actually efficient as much as it is about revenue assurance and scaling for sales / marketing culture organizations.
The scale and reliability is the challenge. You could probably hack out a client and server on the weekend, but simulate 10 million concurrent users and every single part of your stack will break.
Also, I suspect that their strategic plan is not about minimising the number of engineers. They may want to be able to set up non-public projects which iterate for years or months before becoming visible: frameworks, app ideas, you name it. Their Fabric platform gives them excellent insights into what people are doing on mobile apps, for example. The kind of insights and understanding they have into personal and aggregate app usage is quite frightening when you realise just how much they have instrumented.