She scales hard on AD with everything she does. She also works hard on attack speed. IBG applies a slow to someone who won't be simply walking away from you. Unless extreme mobility is used, they can't escape lunge in the first place. People get IBG on ez so he can chase someone down or kite them. Fiora typically won't kite with it and no one is escaping her once she lands the first lunge, especially given she has some slows.
You're missing out on an ad item for an armor item with a nice passive. The CDR and mana Is also nice, but armor doesn't work well without supporting health so you'd need more health to make it matter. Tri force is a lot better if you want a chase, on hit item. Increased movement speed from triforce plus the passive on hit ms boost plus the ms you get from hitting her passive coupled with a couple of slows and possibly even redbuff really means IBG isn't as necessary as you'd think. Plus you get AD, and delicious attack speed along with some nice mana too. And some CDR.