[@Durandal]
They don't use void techniques regardless of whether they know them or not. Some mysticism effects may be linked to items that emit sound, but they would refrain from using these as well. One of The Division's strongest weapon is the anti-sound device, which nullifies the use of mysticism within its effective range. This means that they can't use abilities themselves. If Ferdi has picked up some techniques over the years, then that's fine. Other veteran division members probably have as well. But, again, they don't use them.