It should be possible to install the latest version. The line in the compatibilty file means “Discourse versions before 3.5.0.beta9-dev must use the old version of the theme”. Since you’re on beta9, you should be able to use the latest version of the theme.
I’m so sorry - I noticed the problem yesterday when I first upgraded, but I waited until today to report it. At the time, the dashboard wasn’t showing any available updates for the component and I jumped to the conclusion that it was because of the compatibility file. In fact it was probably just that the background check-for-updates task hadn’t run yet .
I have now updated the component and everything is fine. Sorry for wasting your time!