It's not a "myth."
The problem with making a yeast starter with dry yeast isn't that "you can't do it"; it is the risk of making too small a starter and damaging the yeast. You probably need to start with a 2-gallon starter (1 gallon at the very smallest).
The nice thing about dry yeast is their glycogen reserves are "pre-stored" making them sort of "supercharged." It's the reason why, technically, you can pitch a lower cell count of dry yeast as opposed to liquid yeast.
1 package is probably not going to be enough for a 15G batch, so if you can't afford 2+ or more packages, make a yeast starter. But be sure your starter volume is large enough or you could actually cause the yeast to be less healthy in the starter than they were coming out of the package.