Yes, this is true.....concentric coils shoot the signals in a cone shaped fashion, so you need to overlap the coil sweep to take in all of the ground coverage. With DD (double D) coils, the signals shoot straight down and you cover the entire ground directly under each sweep, without the need to overlap the coil sweeps.
But there are many other differences in detector brands and detectors have come a long way in advancement over the years. Some detectors have developed circuits called Broad Band Spectrum (BBS for short) which were introduced in the Sovereign and Explorer series. They operated on more than just one or two set frequencies. These detects use 17 frequecies simultaneously which results in much more information accurate target idenification.
You can also purchase detectors featuring Full Band Spectrum (FBS for short) which utilise 28 frequencies rather than the 17 found on BBS detectors. These detectors with 28 frequencies are capable of detecting targets much deeper and yet are sensitive enough to smaller targets, and are great for detecting lower conductivity metals such as your gold, brass and platinum. Some of these detectors however, have a great learning curve than other brands and take time, patience and practice to master them.
It pays to do your homework on a variety of different detector brands and models and buy one that's suited to your needs, whether it be goldfields, woods, beach, underwater, parks and reserves hunting.
Hope this helps.
Golden