I fail to see how Google could work around this kind of thing. I mean, you've got nothing flagging the "guest" post as illegitimate other than the fact that it links to an SEO-target, assuming the SEO emailer is telling the truth when he describes non-spammy unique content.
Likely, that's all they could go on - "Your blog has a dofollow link to this site that we know is super-sketchy and now you've been sullied by the link".
Likely, that's all they could go on - "Your blog has a dofollow link to this site that we know is super-sketchy and now you've been sullied by the link".