It is both technically true, and technically untrue.
For the purposes of hobby metal detectors, there is little difference between 3Khz and 45Khz. They will both equally detect highly conductive or low conductive tarrgets. But, in the all encompassing realm of physics, there is a difference. More conductive targets will produce a stronger response at lower frequencies and less conductive targets will produce a stronger response at higher frequencies. This is all due to eddy currents and the skin effects. Is there a significant enough difference in response signals for at a given target at a given frequency to make a huge difference? That answer likely comes down to how the signal processing software is written.
The one advantage of lower frequencies over higher frequencies comes as the ground VDI approaches -95. Higher frequencies tend to produce higher VDI values for a given target opposed to lower frequencies. For larger, highly conductive targets, higher frequencies will tend to cause those targets to wrap into the negatives, where as at lower frequencies, the VDIs will not wrap into the negatives.
So, if you are looking for highly conductive targets in -93VDI ground or lower, you will want to use lower frequencies to prevent quarter sized targets or larger from wrapping into negatives. If you are looking for 14k gold in the same soil conditions, higher frequencies will do you just fine with no risk of wrapping.