The JioPhone can run WhatsApp, but it isn't pre-installed.

For that reason, I'd imagine 90% of Jiophone users in India would install at least one third party app.

However, they would get this from the JioStore, not from the KaiOS store: https://faq.whatsapp.com/en/kaios/26000183/?category=5245235

So if the majority of KaiOS users are using carrier-branded phones with carrier-branded app stores, maybe the developer story for 'vanilla' KaiOS isn't as important.