WhatsApp and Instagram at the point of acquisition were simpler than Facebook is (and was), or even compared to what it is now. Once you scale you start to need a lot of engineers to help keep things standing up and everyone on the same page.
WhatsApp had like half a billion monthly active users when they were acquired, that could be considered fairly large scale, no? But I agree with your point in general.
Yes, but WhatsApp is a point-to-point communication tool with mostly small groups. Each individual message doesn't need to be distributed to a potentially very large audience like in Facebook, making processing and coordination of nodes smaller and simpler.