No, only that he uses the ink; not that he "needs" it.
Harry doesn't "need" his staff, or his blasting rod, or his shield bracelet, or his force rings, etc etc etc.
But he uses them, nonetheless!
We just don't know enough about Peabody; but I suspect it's something similar -- he uses the ink to enhance or leverage or hide or ... () his magic. Maybe to serve more than one purpose!
I think you are totally missing the point here of what Eb was saying. Does Peabody need to use the ink to get inside of minds? He is a fully qualified wizard, secretary to the Senior Council, that means he has skills. A surgeon has the skill to take out a liver without anesthesia, but the patient would be very aware that he was doing it. That is the whole point of the ink,
"revealed the presence of a number of chemical and alchemical substances that are known to have been used to assist psychic manipulation of their subjects." Peabody screwed with the minds of fully qualified wizards, like the patient under the knife getting his liver removed without anesthesia, he'd know it was happening .. If someone is trying to manipulate a full wizard's brain, they know it.. Doing that kind of thing is forbidden, punishable by death. So just as anesthesia makes the surgeon's job a lot easier if the patient is asleep while he removes the liver, the ink numbed the brains of his fellow wizards enough that they weren't unaware that Peabody was tip toeing in their heads, making it easier and safer for him.
Repeat, it wasn't ordinary ink that was found in Peabody's room, it was ink
" revealed the presence of a number of chemical and alchemical substances that are known to have been used to assist psychic manipulation of their subjects." It really doesn't matter whether or not he needed it, the whole point is he used it. Could he have manipulated their brains without it? Yeah, he is a full wizard, he could, but at great risk of detection.. The use of the ink eliminated that risk..
So on the academic question on whether or not Peabody needed the ink to pull off mental manipulation of his fellow wizards? No, any qualified wizard could do that.
However on the question of whether or not he could carry it on for years without the anesthesia of
the ink compromising the free will and awareness of his subjects? No, he needed the ink.. Like liver
surgery, if the patient is awake, they know what is happening.
As Eb said,
" It is my belief that Peabody has been drugging the ink for the purpose of attempting greater mental influence over the decisions of members of the Senior Council, and that it is entirely possible that he has compromised the free will of younger members of the Council outright."Yeah, the ink's sole purpose was to hide and leverage his magic.. And yeah, he needed it, maybe not so much on the youngsters, but on the decision makers, he couldn't pulled it off without it.