Generally, this is true, regarding knife steels vs. sword steels. A sword steel needs more flexibility than some knife steels. There is much more to this subject, of course, and I cannot speak about the characteristics of S30v or ATS34 in this regard. A sword is not just a big knife. The traditional Japanese makers solved the problem by a differential heat treat, with the edge being harder and therefore more brittle, and the spine left more flexible. Many custom knifesmiths do this today, and this practice is also used for axes and other chopping tools. Many of the less expensive swords on the market are left soft throughout the blade, because, of course, they are for wall hanging, or for reenactors to bang blades together. The custom sword shown here may not be differentially treated; perhaps a steel is used that does not require it, or the design itself may be massive enough to not require much flexibility. I am no expert of course, just an interested student of swords.