Is index 0 mean hour 1h and index 23 mean hour 24h (or 00h) ?
Yes. This is how the EPW file datetime format is and we follow that in the legacy plugin. The timestep is from 1 to 24. There is an open issue for EnergyPlus to change this.
NOTE: This behavior has changed in the Ladybug[+] plugin but we can talk about that when it comes up as an issue.