With Bungie you never know if it's actually a key to a Easter egg.
The Stranger hands it to you. This could be Bungie's way of saying this is important for other things.
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I don't know if this works 100% of the time but it's worked twice for me now. Every time the daily has been the black garden, let it play through the mission timer and cut scene at the end, do not go to orbit. After the cutscene you will be rewarded with the strangers rifle again. Whether this work if you load up the black garden manually I don't know. Worth a try. Good luck.