Looking at the license [1], it appears to be GPL v2 with a classpath exception. This means it doesn't directly deal with the issue of patents, only copyright. There have been cases in the past of companies going after users for patents on otherwise open source projects. The worst part is, Oracle reserves those right so they can decide retroactively to go after those users (unless Oracle decides to modify/replace the license).
I think Graal itself is GPL2+Classpath because it's derived from a codebase that was itself licensed that way, not because Oracle actually want ambiguity. If they wanted that, their new from-fresh codebases wouldn't be under a more precise open source license.
[1]: https://github.com/oracle/graal/blob/master/LICENSE