You're seeing just the function loaders, go back to Ecto.Adapters.DynamoDB module for more information.
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loaders(primitive, type)

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Returns the loaders for a given type.

Rather than use the Ecto adapter loaders callback, the adapter builds on ExAws' decoding functionality, please see ExAws's ExAws.Dynamo.Decoder, in this module, which at this time only loads :utc_datetime and :naive_datetime.