I can't tell you why it's not compatible, but regarding the second question -- if an app has ever been installed on any of your devices, it will show up as "Installed" on the Play Store website. If you click the Installed button, you'll still see a list of your devices and if the app can be installed on them.
Also moved this from the Android Apps forum to the S8 forum, since the former is intended for developers to promote their apps.