I'm not an expert, but my understanding is that Presto by itself does not store data. It is a SQL query engine that connects to a wide variety of data stores...relational databases, cloud storage like AWS S3, local file systems, etc. As applications are developed to work in an interconnected world, tools like Presto work as intermediaries for SQL queries.
Since IBM bought Ahana, it stands to reason that DB2 will be well-supported within the Presto framework. That doesn't mean that IBM or its customers will dump DB2...far from it. It's just another open source project (like Red Hat Linux) that IBM has decided to participate in (with significant financial resources).