This was a difficult concept for me also. The key thing is the terminology that is used for 2 very different things. The terms high and low refer to tides which is the vertical measure of the surface moving up and down as a result of the moon’s gravity. Currents on the other hand are a horizontal movement of the water. There really isn’t such a thing as “high slack”. Think of it as “slack when the water is highest or lowest”. It might seem logical that the water would flow horizontally (current) until the vertical maximum or minimum are reached (tides). If you were filling a pool that would work. The water flows in the pipe until the level is full. In the real world the pool you are measuring your tide in has many entrances (pipes) flowing in and out at the same time so you cannot assume the time of maximum flow is the same as the time of maximum height. The rules are if you are looking to anchor or are worried about depth consult the TIDE TABLES but if you are worried about current flow you need to consult the CURRENT TABLES.
They are related but definitely not the same thing.