This is almost accurate, but not totally though.
It's really Daichi Suzuki that first developed this UDK to this extent. In fact, he first displayed this technique during '84 Olympics, but failed to qualify using it.
For the vast majority of people, NO this technique is not the fastest way to swim. Personally, at Fly (I hate back), my UDK is significantly slower than my full stroke (probably around 17.5sec vs 14.0 for 25m). Therefore after diving, I usually pull out very rapidly avoiding wasting time doing like so many others, ie dolphin kicking without even realizing that it slows them down.
Also note that the Streamlined Underwater Dolphin Kick like it's often called, or SDK, is entirely different from the undulation action that takes place when swimming the Fly. Whilst swimming the fly, the undulation should include upper body. Whilst SDKing, upperbody must be fix, and therefore the head shouldn't move at all. Torso must be fix and stable. Actions therefore starts from the abs down to the legs.
It is very very difficult to accelerate this particular action passed your top speed whilst swimming, it takes serious dedication. Having floppy ankles doesn't seem to guaranty success, as mine are very flexible and floppy, but yet my SDK remains significantly slower than my full stroke.
Over the course of my career, I only came across one real back swimmer who mastered this gesture. Not that surprisingly, he still holds our team record over 50 back in 25.07 (try this). It was done in 1995, so in this case, you'd see him popping at the 15m mark both on the way up and on the come back.
His fly was kind of crappy by my own standards, but his SDK was so hot that he could easily qualify for Nationals at 50/100/200 Fly as well.
Also note that whilst a lot of people are in admiration with Phelps Freestyle, I don't think he should even be used as a model at that stroke. Without his top notch SDK, this guy would have been off the chart (below it, and quite a lot).