Maybe she wanted to flush Joe's idol too.
That could be a possibility for sure. it's just hard to see what actually happens when they push the edit 1 way in hindsight knowing the result, trying to get the best suspense. All we heard for Jessica's reasoning was "I want to help the Healers", she could have followed that up with a whole bunch of other logic, but that's the line they wanted us to hear. Knowing that it came to 2 to 2 and Joe played his idol correctly they show that line instead, and played out the how the advantage blocked us into a tie and will Joe play the idol correctly angle rather than getting into the placement of the advantage angle since it didn't make any difference int he end.
Of course we're never going to get the full story, just looking back to the first episode is proof of that. Where Chrissy was more in with the group and wanted Katrina out, we got more of the which older person are they going to throw their votes on and will Crissy protect Katrina with the idol instead of the real angle that was happening. I just wish they'd give us a little more glimpse into the actual logic there from Jessica instead of the 1 liner of logic followed by advantage usage that didn't follow that logic much at all.
That was a tough advantage to play if she wanted to flush the idol but keep the Healers safe though. You force the obvious 2 to 2 tie by blocking Devon's vote and there's no guarantee Joe plays the idol right either. Wouldn't shock me in the least if she wanted Joe gone completely, and "protecting the Healers" was protecting the other Healers from Joe later int he game or something who knows. Hope we get exactly what the logic was there later in the season from exit interviews when she gets voted out or at the finale if she's in it.