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.
Users often get apps installed for them by the store at purchase. So, while they may have 3p apps installed it's not necessarily a good signal they will install more.
From the article, it seems like pre-installed apps have ample usage, no mention of installation post hardware purchase.