Ros from Campaign Monitor here - I've just updated the post based on Matt's contribution above.
It would have been very unwise to recommend that any sender (not just our customers) change how they manage their lists based on an anecdotal report, thus we really did stress that there was no evidence at this stage to back up the claims in the original Lockergnome post.
By all means our customers are welcome to clean, or segment their email subscriber lists as they see fit. However, there are some sound reasons why we don't recommend this - http://help.campaignmonitor.com/topic.aspx?t=54
(To clarify for others, 'inactive' in this context means 'does not open campaigns' - unlike bounces, which do get automatically removed from lists)
That said, list management isn't the topic at hand - it's relationship between reputation and Google rank.
It would have been very unwise to recommend that any sender (not just our customers) change how they manage their lists based on an anecdotal report, thus we really did stress that there was no evidence at this stage to back up the claims in the original Lockergnome post.