About “zuke”, yes it is correct that it is a technique used during the Edo period to preserve food as a result of no refrigeration. However this is the quicker and easier method of soy sauce marination (as it does not need a lot of time) and is the more common approach used by chefs.
The traditional method is more rigorous and exacting, and would result in something that looks more like this
The chef would start off with a section of tuna (typically the dorsal cut, akami), applies a technique otherwise known as “yushimo” which is pouring some boiling water over the surface so it cooks the exterior, then immediately immerse in an ice bath to arrest the cooking process, then marinate the entire block in a mixture of soy sauce, mirin (some add kelp).
The definition of success here, is that you have an exterior that while is cooked, is very small in terms of thickness, and acts also as a barrier to slow down the soy sauce marination osmosis (if you will), versus soaking a raw piece of tuna in a pool of soy sauce. Timing, technique, temperature, and control are vital here. If the cooked exterior thickness is a bit too much, the soy sauce marination would make that part overly salty.