I may have read it wrong. A further google search asking if all indy cars will be hybrid didn’t answer it specifically. I did see one where it said something like INDYCAR, Chevrolet and Honda will be debuting their hybrid cars in the summer of 2024...
I think you are correct that all of the cars need to meet certain narrow parameters. Oh - found the sentence in an article:
“INDYCAR will be using hybrid power units, a collaboration between the racing series, Honda and Chevrolet. The new unit pairs the existing 2.2-liter twin-turbocharged V6 combustion engine with groundbreaking supercapacitor hybrid technology.”
And a capacitor - not a heavy battery.
A trap door for extra foot power would be welcome.