With their current approach, they can share one content authoring environment over an API that splits all render concerns to the consuming client.
Sounds a lot like what HTML was supposed to do in the first place doesn’t it… but not all devices support arbitrary html web browsing, nor would you want to serve a full page to most , so the next best option (according to some) is API plus javascript client for the web side of things.
While I don’t think this path is correct, given the tools we have I do see why it is used so frequently.
Do you need an SPA for this? You have to write some code to handle different devices, I don't think it makes a lot of difference if it's being done in JS on the client side or rendered as HTML and spat out to the user direct?