It wasn't concentrated, but I do recall that barrels upon barrels were entered into the river, so there was quite a large amount that was ingested by the Wise One's despite the lack of concentration.

It could be as simple as a matter of time: the attack took them by surprise, it was timed so that the forkroot would have taken affect, and the Wise Ones would have just imbibed some. Many may not have been asleep for that long, and may have already started rousing themselves in time to realize they were prisoners.