I agree, having led large engineering teams in the past I learned the hard way the importance of keeping your customers involved in the entire sprint process.
Take a look here, may answer some of your questions.
Short answer is there is a private backlog maintained by the engineers that we don't get to see. But they take input via the forum and github.
But kudos to Spark, they've done an amazing job so far and are in the early days of development.