Perhaps let's look at telegram. They have somewhere around 30-40 engineers, and handle most of the issues tackled in this post from i18n, to distributed systems at a large scale. You could also look at whatsapp which I believe was a similar size when they had somewhere around a billion users. Both of these were international, huge in scale and with somewhat tight security.

Specifically let's look at this paragraph: "Businesses that actually care about turning a profit will spend a lot of time (hence, a lot of engineers) working on optimizing systems, even if an MVP for the system could have been built in a weekend. There's also a wide body of research that's found that decreasing latency has a significant effect on revenue over a pretty wide range of latencies for some businesses. Increasing performance also has the benefit of reducing costs. Businesses should keep adding engineers to work on optimization until the cost of adding an engineer equals the revenue gain plus the cost savings at the margin. This is often many more engineers than people realize."

If this is true, then why are the above apps extremely high in performance compared to say the Spotify app which has ~2,700 engineers? Or Github, which again has page loads which are pretty slow by modern standards? I don't really think this is a valid argument.

I think the features argument presented is more of a valid argument, but it's not really an excuse to compromise the core functionality of your app. The amount of times I find Spotify unable to execute it's most simple task of searching (local songs) and playing music despite the huge engineering base.