True. High strength fastener design guidelines tend to agree. My 3-5 threads is based on pitch tolerance studies that show that only approx 3-5 threads of most fasteners actually engage mating surfaces. The longer a threaded engagement, the more likely the condition becomes.
This is one of the major reasons for torque to yield bolts. It gives a greater chance of engaging more threads by stretching the faster into as much engagement as possible.
It also assumes axial load, and no shear. I would definitely stay at 1.5-2x diameter if any shear is present.
In practice, I've never had a design fail in axial tension when following the 3-5 thread rule with coarse threads in like materials. I double it if threading into aluminum or titanium though. 2.5x in magnesium, and like I said: if shear is present, go for no less than 1x diameter (1.5-2x preferred).