You still gain the benefit from having a nice faster GraalVM, and given it's open source, you could create a fork and encourage the community to use it as an alternative.
I'm sure if you were producing a much faster GraalVM, you'd soon get people to move away from Oracle's one to yours (MariaDB vs MySQL springs to mind as a comparison)
The problem here is what loophole a big company like Oracle can find, if they don't like you. They can't prevent you from doing a fork, but can easily sue you for not keeping copyright visible enough in project page README file.
Taking into account amount of resources they have, this will be enough to shut you down ;)
I'm sure if you were producing a much faster GraalVM, you'd soon get people to move away from Oracle's one to yours (MariaDB vs MySQL springs to mind as a comparison)