She explains in the text. They were using her as a first hurdle because she got a good reputation for interviewing. The rationale seems to be that if they put "well-calibrated" interviewers in first, they might weed out time-wasters earlier.

I work at a company with a similar policy on technical interviews (though I don't do any), and here I think it's uncommon to decline a particular interview unless you have some very pressing engagement.