“Most turbo charged piston engines only have compression ratios of 7.5 to 1. The big myth, is that these are high compression engines and have to use 100 octane fuel.”
That’s because the turbocharger is compressing the air before it even gets in the cylinder. The result is even greater peak pressure and power (and chance for detonation).
The piston/cylinder compression ratio may be low, but the overall compression ratio (turbo+piston/cylinder) is even higher.
Which is the whole point of turbocharging an engine.