It's actually on the bottom of the announcement that you get it later, September 16th - but for those who already purchased here at 10.99 it does seem.... unfortunate that you are being penalized for buying it before the steam release. It's a handful of weeks to wait, but no additional cost seems fair.
In effect however, this might discourage people from buying future titles on itch.io at all, if they feel slighted by this.