It's not type-safe of course. Each function taking a Context has an implicit interface (the required key-value pairs that the caller must provide in the context) and failing to provide an expected binding will likely result in a runtime error.
This is why dependency injection frameworks like Guice and Dagger went with a different approach: they provide environment values to a class's constructor using reflection. Then the framework can do type checking at startup (for Guice) or compile time (for Dagger).
However, that has different costs: it requires a class per component instead of a function per component, and the framework is more complicated. This is an engineering tradeoff and apparently the developers of Context decided on simplicity.
This is why dependency injection frameworks like Guice and Dagger went with a different approach: they provide environment values to a class's constructor using reflection. Then the framework can do type checking at startup (for Guice) or compile time (for Dagger).
However, that has different costs: it requires a class per component instead of a function per component, and the framework is more complicated. This is an engineering tradeoff and apparently the developers of Context decided on simplicity.