For the record, the Cambridge ticket loophole was pointed out to ATOC in 1998 in writing, but they appear to have done nothing to close it, so I think its a bit unfair for TOCs to try and Penalty Fare passengers using it. That said, I would not attempt this journey unless you really are confident about your knowledge of the Routeing Guide (even though its written very unambiguosuly for this case) and have spare time on your hands to appeal
They did do something about it (or at least thought they did) by adding new maps an changing what maps you could use for a given journey.
So London - Finsbury Park is now only valid on Map EE (which is London to the North East and West Yorkshire via Sleaford, so most definitely doesn't feature Cambridge), and London - Hackney is only valid on Maps TS (LT&S plus Gospel Oak - Barking, the NLL between West Ham and Willesden Junction and Romford - Upminster) or WA (which does feature both Cambridge and the Great Northern routes via Welwyn and Hertford).
And I would fully expect anyone attempting to use a London Terminals - Hackney Downs ticket at Kings Cross, Moorgate or Old Street to be told they need to go to Liverpool Street, and anyone en route to, or arriving at, Cambridge with one to be PFed for travelling beyond the validity of their ticket regardless of what the Routeing Guide says.
Cheers,
Barry