The purpose of the 72 hours darkness is to increase THC % in the already existing trics, it's doesn't add more trics! Your tric number is a genetic factor.
You are right about how an extended period of darkness increases the amount of THC in existing trichomes and not causing the creation of more/new trichomes.
But what you said about the number of trichomes being; "a genetic factor" is only so accurate. The genetics of any strain might be called it's upper or highest limits, it's maximum possible level, but there is no guarantee of ever reaching, or even getting all that near to the max, so someone could grow a strain that could be the whitest ever and it not end up being all that white.
Genetics are about 50% of your final result, with the other 50% being made up of the very best possible growing environment/conditions and grower skill.
An example about trichome production. The very same strain, and as in clones, grown in regions of higher UV-B rays will produce more trichomes than the same plant will in regions with lower amounts of UV-B.
So, indoors, if someone adds UV-B lighting they will see an increase in number of trichomes over and above what they would have had, or had in the past when growing the very same strain. All because the growing environment/conditions were more conducive to bringing out more of what the strain's genetics would allow the strain to do or reach.
What it shows is that genetics are not a guarantee that someone will reach/have/achieve the max the genetics will allow. But instead genetics are only a guarantee of having the possibility to reach/have/achieve IF you provide the best possible growing environment/conditions and have the skill needed to tickle out every last bit of what the genetic coding will allow.