Your first thought when reading that headline might’ve been _WebAssembly. Mine was! But no, that is not how Browsix does its thing:
Browsix is a framework that bridges the considerable gap between conventional operating systems and the browser, enabling unmodified programs expecting a Unix-like environment to run directly in the browser. Browsix does this by mapping low-level Unix primitives, like processes and system calls, onto existing browser APIs, like Web Workers and postMessage.