I wouldn't say it's "proof" of anything. Of course, it wouldn't be illogical to think that it's all we'll get. I'm on the fence myself.
Logically, it would be they released this track to begin hype for a new album— but why this mediocre song? Maybe it was all they had ready and wanted to begin advertising. But like was said before, this is Gn'R and logic goes out the window.
Also, you're assuming that Axl hasn't done ANYTHING since those 2000 leaks. We know he did because some of the tracks that made it on CD are much newer. And I'm not defending Axl. I'm as disappointed as anyone else with the lack of releases. But I think his issue is more his incessant tinkering and insecurity than his lack of re-doing vocals.
I'm sure there are plenty of other vocal tracks - it's just a matter of whether Axl will get over his fears and insecurities and give us something completely new. So far, the outlook isn't great. Of all the songs we could've got and he could have released as a single and "debut" (or re-debut) live, Silkworms? Hmmmm... I can't see how Slash and Duff could've thought, "Yeah! Let's release Silkworms, everyone will love it!"