Currently the JUnit5Runner checks for annotations with name.startsWith("org.junit.jupiter.api.") to support any JUnit 5 Engine, e.g. Spock 2.0. This should be extended to check for the presence of the Meta-Annotation @Testable instead.
So check for the presence of @Testable directly on the method/class. Or on its presence on other annotations, e.g. JUnit Jupiter @Test is annotated with @Testable.