During the 2000’s, FreeBSD had a more performant tcp stack and many high-traffic websites used it. Considering that Whatsapp ran on a small set of big servers, picking FreeBSD certainly doesn’t seem out of line. I’m not sure if Linux tcp stack has ever surpassed it. For instance, Netflix still uses FreeBSD to serve video.
https://papers.freebsd.org/2022/eurobsdcon/gallatin-the_other_freebsd_optimizations-netflix/
https://papers.freebsd.org/2021/eurobsdcon/gallatin-netflix-freebsd-400gbps/
