Well, your example invalidates the “other than turbos” part because the turbo is responsible for probably 80% of that power gain. So then the question is, why is everyone using turbos now and not before? Any competent engine builder could get 500hp out of a 3 liter turbo motor back in the 80’s, so what changed?

The big advancement is reliability and emissions. This is mostly driven by engine computer simulation driving better engine geometry design to help these high power engines burn cleaner, but probably the biggest factor is ECU technology and sensors. If you look at motorsports, drag racing in particular the advances in how much power those cars are making has continued right along with OEM cars getting more powerful. Faster ECUs that have faster sampling rates, along with individual cylinder sensing and individual fuel injection means that each cylinder can be run at its absolute peak performance. In the past you would have over cylinder that’s the hottest one, and that would be the first to detonate. Since engines used to be tuned all at once, all the other cylinders would have to be detuned to the same level, plus other environmental factors could not be controlled so the engineers would leave some margin for adverse conditions. With that latest tech, ECUs can detect each firing event and individually adjust fuel and ignition for peak performance without sacrificing reliability over a wider range of conditions.

The materials science has improved as well, especially for turbos, but the big advancement has been in the ECU and sensing.