Yes, entering a speed on the VNAV descent page effectively creates the first speed constraint.
Although the FCOM splits the speed logic into situations "above and below the first speed constraint," it may be more useful to think of it this way: if the FMC is allowed to choose it's own target speed, it will choose a new target speed upto Vmo/Mmo -11 kts in order to stay on path. If, on the other hand, it has been programmed to maintain a speed chosen by the pilot, or dictated by a terminal procedure, it will only deviate up to 15 kts from that speed in order to maintain the descent path.