It's true they lacked the understanding because the fresh wet pus would easily transmit the disease compared to dessicated product. A lot of work for no reward. But... they were also trying to avoid getting sick, themselves, while likely playing in early biowarfare on behalf of someone's government. I'd even go so far to say that much of the plague, at this time, was actually staph infections causing the boils and they could have easily transferred this via skin to skin contact.