It just upset her even more that he thought he could just spill his whole strategy like he thought he'd still win out in the end, even if she knew. He always did, though. He always told her, and still she always stubbornly pressed forward assuming she could triumph through sheer force of will. It was how she'd always done things, only this time, she was upset enough to be sloppy.
Buffy didn't say anything, responding only with a heavy-handed and wide-arcing punch that was both reckless and strong. It spoke volumes of where her mind was at right now and that he was clearly getting to her. If it connected, it would hurt, but while not tiring, her own emotional state was causing her to leave herself wide open.