At my previous job, it wasn’t just about you being seen as idle, the management was also tracking what time you left a “timestamp” by performing an action on files, in software, etc. So a jiggler would have done no good.
It’s possible that the bank was able to tell that even though the workers never went “idle,” they went 1-2 hours without entering into a customer’s bank account and performing any work, or something of that sort.