(Modifying answer based on additional information from the OP)
Looking at The Scions of Shannara, at the beginning of Chapter 11 (This is page 135 in the Del Ray/Ballantine edition published in 1990), it says clearly:
Brin had encountered them when she, Rone Leah, Cogline, Kimber Boh, and the moor cat Whisper had gone after the missing Sword of Leah
It says very clearly "Sword of Leah", not "Sword of Shannara". This fits the story to which it is referring (see below). So, if your edition says "Sword of Shannara", it is a misprint.
This is referring to Wishsong of Shannara. Brin encounters the Spider Gnomes when traveling with Rone Leah, Cogline, etc. They were, indeed, on the quest for the Ildatch, but at this particular incident (around page 368 of the edition I have handy), they are trying to retrieve the Sword of Leah (not the Sword of Shannara), which was lost earlier,
when Allanon died (if I'm not mistaken).
Rone was certain they must have the sword to succeed in the quest.