I'm suspicious of anyone who seems to have trouble understanding the FBI's claims. It's clear to me that what the FBI is claiming is that some part of the CAPTCHA was linked to the server directly, rather than its TOR address.
Even in the hypothetical case where – for some unrealistic reason – the Silk Road hidden site was including an image on an external server by referencing its IP address or hostname, the agents would still observe this traffic as having come from Tor. There is no magic way that the traffic from a real IP embedded within the HTML of a hidden service would find its way directly to a client without passing over the Tor network and through Tor nodes. Were this the case, it would be a huge vulnerability in Tor, as it would allow the administrator of a hidden site to uncover visitors by including an element that is served directly to the client over clearnet (thankfully it isn’t and this doesn’t work – try it).
