It is indeed a lot, but since Whatsapp manages to get 10 million connections per box [1], 100k seems doable even without insane wizardry. A few years back, I wrote an article on my blog at https://wjwh.eu/posts/2018-10-29-double-hijack.html about keeping 65k connections open in a single Ruby process and that was a hackathon project written in 4 hours.
https://medium.com/codingurukul/whatsapp-engineering-inside-...