The entire point of that line is that they were wrong and they were blind. Clearly, Sidious and Plagueis (and now Maul) had been around forever. So one reading is that were just completely unaware and truly didn’t know. That can still happen (notice they haven’t gotten the real Jedi council involved yet for a reason), but there are multiple other possibilities rather than taking that line at 100% face value. The #1 lesson from the prequels is that the Jedi were completely blind and not always innocent themselves.