This comes from the fact you most likely did not implement the advised robots.txt at first.
It's no big deal, here is how to fix this :
1) drop the post line in your robots.txt in order to allow post urls again (I know they are dupes, but we'll take care of them in step 2)
2) Install activate the zero duplicate and set post redirection to at least guest (none by default).
This way is faster because Google will continue to crawl post url and will get http 301 redirected to topic urls, where if you keep the disallow, it will not acknowledge the redirecting since it will not (or only very occasionally) crawl these anymore.
After couple month, post url will have disappeared from Google index, then put back the disallow on post urls, this will save some useless redirecting for bots.