An obvious question is whether the focus on performance first is correlated with the success of stack overflow. If the same level of succes could have been achieved without squeezing as much performance out of each server then the sacrifice of testability is a dubious one.

The article doesn't explain why horizontal scalability wasn't pursued instead of their approach of performance efficiency (i.e. greater performance from a given amount of resource). It's certainly true that at the time that SO was starting, scaling horizontally required far more handbuilt tooling than it does today but it certainly was already a widely used pattern. The likely reason is cost, but that then comes back to the first question - would SO have been equally successful if it had higher infrastructure costs in its early days?

The persuit of efficient performance at the cost of aspects of quality (of which testability is only one) can only be justified if efficient performance is truly necessary for success. A much smarter person than me warned against premature optimisation. That seems relevant here.