What’s interesting is that in Google’s example of a problem page, Google accurately identified the one guest post out of a total of five outbound links in the article.

That meant that Google was able to exactly spot which of the links was benefited the person making the guest post.

No Official Statement from Google

As of this writing there is no official word from Google.

Google’s manual links warning asks the publisher to add nofollow links to the guest posts. In my opinion that suggests that there’s nothing wrong with accepting guest posts or asking to publish guest posts as long as there is a nofollow link attribute on the link.