From the previous article in the series:

> Our problem is further complicated by the fact that we’re not shipping an app but a framework for others to use in their own apps. We have to be mindful of the impact our technology choices have on our customers, such as the potential for harmful interactions with their code, and user-visible performance impact in terms of CPU and memory usage.

https://bionic.fullstory.com/rust-at-fullstory-part-1/

Without knowing the details, framework-as-a-product argues strongly for a native-only approach to me. Xcode + ObjC/Swift on iOS and Java on Android.

The article(s) might make a better case for polyglot by talking in detail about what the framework does in more detail. The bar for going in that direction would be very high for me.