The claim is true if the injectors are of good quality (more below), but please consider the source of your new injectors very carefully, because injectors are extremely critical to the way your engine operates, and you need good quality nozzles, not cheap knock-offs.
With a diesel engine, thermal efficiency is best if combustion happens as close as possible to top-dead-center. If you could jam all the fuel into the cylinder in one short shot, that would give best efficiency. Extending it over a longer duration extends the burn later into the power stroke, turning more of the fuel into heat and less into power. So, you need big injectors and a short injector duration to move towards this ideal.
Crappy spray patterns from inadequately machined holes or worn needles due to improper hardening and surface treatment will, of course, turn that extra fuel into smoke rather than power. Once again, be careful about where those injector nozzles are coming from.
Another source:
www.kermatdi.com
I will defer discussion about the source that you originally listed to others ...