Firstly, I’m not really sure where you are getting your figures. There were 200,000 paid members under the previous leader and it went up to 600,000 just before he was elected.
Secondly, it seems like you’re attributing this sharp increase to a third party nefarious action. I would assume that it were simply a larger portion of those 10m voters deciding to register membership in order to vote in a leader more in tune with their party values.
I take the point that a small group only needing paid membership to vote is open to manipulation. However, I don’t really see a comparison between these two events.