I too am having the same / similar issue with authorizing / linking Stringify to Google Home.
In the stringify app on Android, I go to add the Google Assistant thing. I'm then presented with the "Connect" screen and tap the button. It then takes me to a screen with 4 steps to "Enable Stringify in your Google Assistant", stating in Step 2 to "Launch the Google Home app", and a link button will be presented.
When I click the "Launch Google Home" it takes me to the play store, when the apps is already installed locally. I click the "Open" button in the play store to open the app, but I'm not presented with a card to link to / authorize Stringify to Google Home / Assistant.
Anyone have any thoughts as to what I'm not doing right?
Update: I just tried following the same process on iOS (iPhone 6S+ and iPad Air 2), which also met with the same results as above [took me to the App Store (not the Play store - of course) ].