The interim system may have been detected simply through routine pattern analysis of the internet data.
Too many “mays’ and “maybes”.
The KISS answer is that Chinese security have the best hackers, and were able to find these “breadcrumbs” through Hillary and ALL other “private” servers running around DC.
It sure is strange that multiple high level government officials, ALL with TS or SCI designations, and all within the Obama administration, were set up so close together and emailed each other so often.
There is a reason that American security used to be strong, and is now not strong - elite privileges allows to the Pigs More Equal.
How is that KISS? Are you saying they hacked into Hillary's server, read emails with names of agents, and rounded them up? That's not KISS and there's no evidence for names like that being in any of the emails.
Here's KISS: some crappy CIA software connected clients back to a "secret" proxy somewhere. The Chinese figured out that the proxy belonged to the CIA, which can be leaked (KISS) or figured out (not quite as simple but doable). Then anybody connecting to that proxy would be rounded up. Sometimes that would require arresting everyone at the internet cafe and going through all of the computers, but that's easy for Chinese Communists..