They were acquired for 19 billion dollars, so they were very obviously doing things right. Your condescending snark seems misplaced.
Maybe the fact that WhatsApp was colossally successful, more so than nearly any other startup -- this was the largest ever acquisition of a venture-backed company -- should get you to update your priors on what kind of scaling practice is a "good idea" and what isn't.
If you don't mind, I'd rather give a different view of WhatsApp and on-call.
It quickly turns out that WhatsApp could have a nice exit, so it made sense for some employees to sacrifice a bit of work-life balance, instead of looking for another job.
Your mileage may vary depending on "How many direct phone calls from customers does your company get when the service is unavailable for 10 minutes? and how much money is lost?" We don't have this data on WhatsApp.
Do you have experience in 50 people startup growing like mad and going worldwide? I've been there and I've seen how bad it can be for people responsible for the critical pieces of the tech. WhatsApp is no magic. Lots of hard work and elbow grease.