The mention of Jigsaw is a red herring. Jigsaw, after long years of design-by-committee, suddenly devolved into a proxy war between enterprise players invested in existing module solutions. It's not about making parts proprietary at all; rather, it's about protecting existing investments in modularization ecosystems, the kinds of which form an important part of several companies' business models, all interspersed with legitimate technical and practical concerns about how to be prescriptive without being overly restrictive. The concern whether parts of a Java implementation can be made proprietary doesn't depend on the Jigsaw question, because there are several Java implementations today that are proprietary in part or whole, ranging from mediocre to marvellous, and they still manage to attract clientele.
However, I do agree that Kotlin is a signal to Oracle. It's a signal that Google has given the Android programming question, and the Java question a good amount of thought, and may have found a way to slowly transition off of a technology whose use literally resulted in them being sued, outcome notwithstanding; with Kotlin they can leverage compatibility with the lowest levels of the Java platform (and thus, all existing Java-coded Android code), give people a modern language that offers niceties and high productivity -- all courtesy of a partner whose interests align well with those of Google, and with whom they have cooperated before.
With Kotlin, Android can migrate off of Java without throwing away their existing investment in the body of code that happens to run on Android today, and do so in a way that makes it likely that developers will migrate voluntarily, vs. being forced.