Would agree that the prevalence of this likely very low, but I don't think anyone can credibly claim there hasn't been a "single attack" as we simply wouldn't know. That said if you think about the different types of attacks in a relatively simplistic way there are targeted attacks (e.g. looking to compromise a particular phone/individual) and widespread attacks (cast a wide net and hope to catch many). A USB compromise is not particularly good for either. It's difficult to target with this type of attack unless your target always uses the same public charge port that you might be able to compromise, but if someone has the ability to discover this and pull off the physical hack they probably have easier access to other tools to compromise a target. As a widespread attack the number of people that will use a charge port is very low compared to other online attacks and again, there are more cost effective ways of compromise here.

That being said, the "cost" of negating this attack is to simply use the charger you already have in your bag and plug into an outlet rather than using a USB port. Pretty small cost here as well.