It's really about bringing any language that can target LLVM IR to the web, and that's more than just C++.
https://en.wikipedia.org/wiki/LLVM#Front_ends
C++ just happens to be a popular choice because there are many useful C++ libraries, and the compiler already produces very optimized code, so the performance benefit is most noticeable. In particular, I'm excited to get the Lua interpreter, written in C, on the web in a performant way.