The short answer seems to be: No.
"The Fitbit API" is an HTTP OAuth rest api, which allows your server (or http-client) to exchange data with the Fitbit servers.
To the best of my knowledge there is no other API or official way to get to this data. Also fitbit does not currently write to the HealthKit Api, so it is not possible to get step data that way. According to this article:
https://gigaom.com/2014/12/24/how-to-use-a-fitbit-with-apple-health/
there should be some workaround, but I have not tried this trick, and it might have other disadvantages and require your users to install other apps on their phones. I imagine that the dataexchange between the fitbit and the phone through bluetooth would be encrypted but that is purely speculation on my side :-)
We have chosen to use the official fitbit Api which has excellent documentation and is rock-solid. Good luck in your endeavors!