If you lower a car, you must increase the spring rate. Or else you'll hit the bump stops constantly.
To say they don't increase wear/stress on the factory struts is misleading.
Now, if they stated they are only marginally stiffer, yet softer than other brands, perhaps there is some truth to the original statement.
Either way, buy new struts either Koni or Bilstein, as you don't want to do the work and alignments twice now do you?