In your example on the Readme:

# Query based on the keys
Book.query(isbn__begins_with="12345")

As far as I know, you cannot use a query on a DyanmoDB table without using an exact match on Partition(Hash) Key.
https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_Query.html
How are you using isbn__begins_with="12345" here?

I tried a similar example locally and I get the following Boto3 error:
botocore.exceptions.ClientError: An error occurred (ValidationException) when calling the Query operation: Query key condition not supported

Is this a bug? If not, why does it work?