I find that, too, is becoming less true.
I am trying to get my shepherd’s purse thread reopened for a similar reason to what is being discussed here, viz., I have a hypothesis that there is more diversity in introduced populations than has been realized. People tend to see what their field guides tell them to see; if the field guide says that there is only one species, then people will see only one species.
That is also my big objection to today’s photo-based field guides instead of the old Peterson line drawings: if the field guide chooses to depict one species of a genus that has a number of similar species, people will tend to identify every plant of the given genus as the pictured species.
So the next question re: dandelions is: who is going to take the trouble to bring the European keys to other continents and parse out the introduced, weedy populations? The British Columbia paper is a good start, but that is just one province of one country on one continent.