Both options live in Settings under Data, in the danger zone, and they do different things.
Delete All Data
Removes everything in the library on this device: notes, folders, chats, tags, drawings, flashcards, quizzes, mind maps, mini apps, the search index, version history and attachments. On iPhone and iPad it also takes all of it out of Spotlight straight away. Your settings and your account are kept. If sync is on, the deletion propagates, so the cloud copy and your other devices empty too. This is the fresh start option.
Delete Account
Appears when you are signed in. It deletes your account, cancels the subscription and removes your synced data from the cloud. The notes on this device stay where they are, so leaving the service does not mean losing your library.
From the website
If you no longer have the app installed, request deletion at grape.cool/delete-account. Enter your account email and Grape sends a one time confirmation link that completes the deletion.
Before either one
Take a manual backup or an export first if there is any chance you will want the content back. Deleting all data is not a trip to Recently Deleted, it is permanent.
Signing out is not deleting
Signing out just disconnects the account. On desktop and on the phone it asks whether to keep or delete the notes on the device on the way out. Deleting there goes further than Delete All Data, taking the settings and the backups as well, and it never touches what is in your account. See Sign in and sign out.