The comment struck me as odd, too.
However, it also prompted a series of thoughts; Jaina has been directly involved in most of the greatest battles Azeroth has ever faced. Thalyssra, although ancient, has essentially been safe in a bubble for thousands of years. They used magic and researched the arcane, but they weren't exactly "honing their battle prowess". That, coupled with Jaina seemingly *unusually* powerful (actually, Sylvanas also seems unusually powerful lately), I think paints a scenario that Thalyssra probably wouldn't want to engage in a direct confrontation.