I don't think this guy understands much about how corporate networking works. If he's a developer, I suppose he doesn't need to, but maybe he could check into details before writing:
"isn't it worthwhile for Facebook recruiting to prepare for such a case, and make the interviewer able to switch to a different Facebook IP range, to give Skype routing a second chance?"
Yeah, um, I don't know of *any* company that would set up their HR staff for such a scenario. And this interviewee doesn't seem to understand how Internet routing works. Assuming the particular Facebook HR office is BGP multi-homed, the Facebook NetOps staff would have to determine what IP address the interviewee is connecting with and then modify BGP local-preference for that AS number to use a different ISP outbound. Or, have two networks with different outbound ISPs available at every desk with staff trained to switch between them.
It would be utterly ludicrous to do something like this for an interview.
What's surprising is they didn't try another option like Google Hangouts. Perhaps as a Facebook competitor, it isn't an option. Did the interviewee not have a speakerphone? He mentions being unable to both type and talk... I think they solved that problem in the 1980's.