It's still not that difficult to trace an IP back to a person. Besides the private marketing databases you can tap into, geoip, crappy ISP support, DNS cache poisoning, phishing, consumer internet router hacking, web service hacking, and other attacks on an address space itself, there's attacks in a social space that give away much more.

Saying this is a really bad vuln because you aren't as private as you think you are online is like saying lock picking is a really bad vuln because you didn't know door locks could be opened by anyone with a bent piece of metal or a shaved down key. Locks don't actually keep bad people out. They just make people feel safe. Same with this idea that your IP is anonymous. It's really not. It's a literal address.