As far as I know Telegram has around the same number of engineers even today, handling around the same size userbase.

That ethos runs through everything the WA team does. I learned a bit about it when I was at fb and I was beyond impressed by how few servers WA used for its core infrastructure. It's a really well engineered product.

What more do you need?

That's still plenty of people if you got a few thousand people at AWS/Azure or in this case Meta's in-house cloud making sure your service is running and scaling as it should.