You don't really need clickhouse in this pipeline. And it's interesting (but obvious once said) that you can replace kafka with s3...at least in the "scalable and reliable destination for lots of data" dimension.
It's a demonstration. Kafka is a very popular input for Clickhouse. So are data lakes. The point is neither need Kafka at all.
That's how far s3 has come - it's replacing products that were the pinnacle of scalability back in the day.
I have to look a bit, but it could probably replace Cassandra now as well.