That supported tab only means that the plugin is supported by someone, in presh case is inflow. The version range of the plugin is updated by the vendor when he uploads the jar. As this plugin was released in 2013 this is why the version is up to 5.1.5 (it only have one version so is quite old ).
As for the compatibility I installed the plugin on a fresh Confluence 5.8.4 and had no problems during the upload process, however you would need to test it locally and see if the add on will work properly.
I would advise you to send inflow an email asking about that:
401 E. Sonterra Blvd. San Antonio Texas 78258 United States of America firstname.lastname@example.org
However when I tried to reach the vendor page it was unreachable :
Based on that and the fact no newer versions were added for more than two years I don't believe you will be able to reach the vendor that easily....
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