I, for one, am pretty sure the article is wrong there. What I've heard from geologists is that Yellowstone's magma chambers simply are not molten enough to erupt.
Right, "up to 20%", whereas the threshold for "barely eruptible" is in the 50% range, according to this article: https://www.pnas.org/doi/10.1073/pnas.1617105113 Unless the rules are way different for supereruptions, which I guess we have to consider, but probably not 30% different.