Producer Partition Count override not effective - apache-kafka

Question

Interpreting this - https://docs.spring.io/spring-cloud-stream/docs/current/reference/htmlsingle/#_producer_properties
my understanding is that, if the partitionCount override is less than the actual number of partitions on an existing kafka topic, then the producer should use the actual number of partitions rather than the override value. My experience is that the producer uses the partitionCount value regardless of how many partitions ( > partitionCount ) are actually configured on the kafka topic.
Ideally, I would like the producer to read the number of partitions on a pre-configured topic from kafka and write messages across all available partitions.
Spring-Cloud version: Finchley.RELEASE
Kafka Broker version : 1.0.0
application.yml:
spring:
application:
name: my-app
cloud:
stream:
default:
contentType: application/json
kafka:
binder:
brokers:
- ${KAFKA_HOST}:${KAFKA_PORT}
auto-create-topics: false
bindings:
input-channel:
destination: input-topic
contentType: application/json
group: input-group
output-channel:
destination: output-topic
contentType: application/json
producer:
partition-count: 2
partition-key-expression: payload['Id']
So, I am expecting that if the output topic is already configured with 6 partitions, the producer will recognise this and write to all of those. Could someone please verify my interpretation above? Or point out what I am missing to get the desired functionality?