Yep-the stuff around them is just mineral wool. Really just a nice medium for the cuttings and small plants to strike in before shipping on.
Roots tabs are an excellent way to supplement heavy root feeders such as Echinodorous or Crypts, which can be very demanding on the substrates. Both I've found respond very well to them. A very good brand are the Ferka ones, which you can buy at Aquaessentials. I've used both Stemma and Rosetta (stem and rosette plants respectively) and they are excellent value and give good results. I've also used Dennerle's E15 as a root tab as well as Seachems tabs. I'd recommend them for use even in plant-specific substrates.
Nick