You are only sharing half the true picture, you have completely omitted the difference in compression ratios between back then and now. At current compression ratios a “naturally aspirated” engine cannot run over 10% without mechanically raising the compression ratio. Turbocharged or supercharged engines at current compression ratios you might be able to “get away with”, but not naturally aspirated engines. Current ratios are commonly 7.5 to 1. We would have to bring ratios up to 10 to 1 or 12 to 1 to run a higher alcohol content than 10% without turbo or supercharging. Alcohol race engines can be 14 to 1 compression + supercharger boost.
So how are these cars currently using E15 and driving around normally?