Mueller just said that neglecting to report to the FBI any information of foreign nationals offering information to your campaign constitutes crime
No, it's not. No more than talking to anyone else is. If they are working for you, you're paying them, or other financial engagements, yes. But, generally, you're free to speak with whomever you like even if they're Russian.
The original dossier was constructed by information from a UK national -- if this was the case, you should have had an investigation for Hillbot too.