HJ, that is not true. You can not backwash all the resin, or any other mineral, out of a tank. That assumes the correct freeboard for the volume of resin/mineral.
You do backwash some resin/mineral out until the water temp is as cold as it gets, then you can not backwash more out. Cold water is 'thicker' than warmer water. That assumes no top basket as Speedbump mentioned.
With a top basket you can only get 'fines' (broken and very small beads) out through a top basket. That assumes you have the right top basket for the type of resin in the tank, or we could say, the right resin for the top basket. I.E., fine mesh resin goes right through a regular mesh resin top basket but, regular mesh resin will block up a fine mesh resin top basket and many fines will not be able to be backwashed out as they are supposed to be for all resins/minerals. Then you have a higher than normal pressure loss across the softener/filter
Also, as to losing capacity, no softener should ever be sized, or set up, to use the highest salt dose for the volume of resin to get the highest K of capacity. If it is, you get terrible salt efficiency. Doing so is like letting your vehicle run out of fuel before you refuel, or running an engine at full throttle constantly. I size and set up for very high salt efficiency which means we leave many K of capacity in the tank when it regenerates. It's like leaving a 1/4 tank of gas in your vehicle when you refuel. You buy less fuel but didn't waste the "left over" fuel in the tank.
The softeners I sell have a gravel underbed, which means the level of resin is a bit higher in the tank than the same softener without a gravel underbed. That reduces the freeboard slightly. So it is normal to have some resin, maybe up to a quart go to drain initially. Any softener with a gravel underbed will, the vast majority of people will never see it, including the guy installing the softener. And Kent is in ID with very cold well water at this time of year. A quart of regular mesh resin, at the maximum of 15 lbs/cuft, might have 100 grains of capacity. It's like a little bit of steak being trimmed off with the fat, you never miss it and getting rid of all the fat is very good for your health; this is good for the softener's health.