It's 2021 and there are 3 major desktop and 2 major mobile platforms, plus some smaller ones on each side. And then there is the Web, which is both mobile and desktop focused.
If you're going to write a new library that you want to use on all of these platforms, you need a programming language that works on all of these. For iOS Java is a non starter because as far as I know there is no proper JVM that can be used in App Store apps. C can target all these platforms (including the web using WebAssembly) but it is a major pain to setup the tooling for every platform, especially if your library needs complex dependencies for things like SSL.
I am not a Rust zealot that advocates rewriting everything in Rust. I only mentioned Rust once in the whole article so your "obviously" comment doesn't really hold. The reason I mentioned Rust as a language for my project is because it is one of the modern languages that have cross-platform building and dependency management builtin from the start. It is a lot less work to write an SSL-enabled library in Rust that can be used on all these platforms than it is to write one in C.