Earlier quoted context omitted.
Generally, at any scale over ~500 computers that run continuously, investing in private infrastructure is going to be 1/2 to 1/5 the cost of public cloud, including people costs. The real levers for improving engineering spend are around complexity of your actual serving software, which doesn't depend on / isn't really influenced by who you buy your computers from.
I don't buy that claim at all. No way you're building private infrastructure for 1/5 the cost of Azure or AWS. Or even 1/2. If you're comparing some servers in a 4th tier colo with a poor SLA to AWS and pretending that devops time is free, then sure. But if you're actually building something with comparable reliability and availability, then no way. You're going to spend a lot more than you'd expect on extra capacity…
Your SLA will likely be better than with public cloud.