DailyDrop has no account system and no user database. When you sign in with Google, your saved articles and settings are stored in a private DailyDrop folder in your own Google Drive. Deleting that folder's contents is what "deleting your account" means here. You can do it in three ways:
Sign in with the same Google account you used in the app. This page will delete your saved articles and settings from your DailyDrop Drive folder, then sign you out. It can only see DailyDrop's own folder, nothing else in your Drive.
Open the app, go to Settings and tap Delete my data. This also clears everything stored on that phone.
Go to myaccount.google.com/permissions, choose DailyDrop and remove access. This stops the app from reaching your Drive, but may not erase data already saved, so also use Option 1 or 2, or in Google Drive open Settings > Manage apps > DailyDrop > Delete hidden app data.
Email dailydrophelp@gmail.com with the subject "Data deletion request" from the email address you use with DailyDrop. Because the data sits in your own Drive, we will walk you through Option 1 or 3 rather than deleting it ourselves.