Great question and I updated the article. When you are testing Strapi it created a SQLite database. When you deploy on Heroku with a Postgres database it will create the schema but you lose the data from your SQLite database. You need to migrate the data of the SQLite database to the Heroku Postgres database or start fresh. You can check out the Strapi forum for more information

https://forum.strapi.io/t/has-anyone-successfully-migrated-data-from-sqlite-to-postgresql/4517

I haven't done this myself but a work-around could be using a Strapi import/export plugin. On your local environment you export your data and on your Heroku environment you import the data https://www.npmjs.com/package/strapi-plugin-import-export-content