I do take issue with some of what they say. Softened water is more conductive so the anode simply reacts and is used up faster. I would not check the anode in three years, but rather first time in one year, to see how fast it’s getting used up. In normally softened water, anodes last roughly two years. In over-softened water they can be down to a bare wire in six months. That’s bad 😯
Do have a look at how much you soften the water. Never soften down to zero grains of hardness as some say, but rather leave three to six grains of hardness in the water as suggested by NACE, the National Association of Corrosion Engineers. A little hardness protects the plumbing.