Quickstep, nowhere in your quote does it say that the leg should be straight. It says that it is only straight at the full extent of the stride, but that's a prohibition against having it straight at other times, not a requirement that every stride extend to the point of straightness. Every stride has an "extent" but not every extent of stride employs straight legs - primarly, only those subsequent to body swing will, while the initial (heal lead) action will in practical terms retain some bend to the knee.

As for videos, those on this site show some overextended strides, and if you look closely the problem that results. If you look even closer, you'll see that despite overstepping the body motion, the legs are still not straight at the full extent of the heel lead stride, and appear to bend substantially before the weight arrives.