Google Assistant gets released in the Play Store, but it's just a shortcut of the app4
It's just there to let you know if it's compatible or not with your device and possibly to get faster updates. It has become standard for many smartphone companies to release specific apps in the Google Play Store to allow them to update them faster without having to wait for the next Android update.
Google Assistant acts as a shortcut to the Android application that's already installed on a smartphone, so it's not really meant to be downloaded on a device. As Google notes in the changelog, this is the first Google Assistant release, so it's likely to continue to update the app.
The description of the app in the Google Play Store notes that Google Assistant offers users another method of launching the personal digital assistant, nothing more, nothing less.