First TIOBE is driven by search traffic which is driven really by activity not popularity. That means that even say, masses of people migrating away from Groovy to Kotlin would generate huge search hits which would cause it to rise in TIOBE. (Note: I'm NOT saying that is happening, just that TIOBE doesn't rule it out).
But I also don't really agree with your conclusion that because Gradle and Jenkins aren't new that they could not be responsible for a rise in popularity. I think those are both ever growing projects (especially given Android depends on Gradle). So you get a continuous stream of people encountering Groovy.
Finally, yeah, there aren't too many high profile Groovy projects, but it is very pervasive as an embedded scripting option, and there are some new projects (eg: Micronaut) where its a great option. It can be found in a lot of places you might not expect (for example, I found out that Samsung's Smart Things are all driven by Groovy DSLs). It's also very powerful for full application dev, which I use it for. I think people underestimate its usage because it so often lives under the radar inside Java or other JVM projects. It certainly isn't too popular on HN, but HN is a very specific ecosystem and intersects very poorly with the "enterprise" world where Groovy would have a lot of its use cases.