SAT 1400+ · Competitive scorer

For two centuries lichens were classified as a partnership between exactly two organisms, a fungus and an alga. In 2016 a team sequencing lichen tissue found a second fungus present in specimens from six continents, including species whose appearance had never been explained by the two-partner model. The finding did not overturn the partnership account so much as complicate it: the alga still supplies sugars, the primary fungus still supplies structure, but the third partner appears to shape the form the lichen takes. Which finding, if true, would most strongly support the claim that the second fungus shapes a lichen's form?

For two centuries lichens were classified as a partnership between exactly two organisms, a fungus and an alga. In 2016 a team sequencing lichen tissue found a second fungus present in specimens from six continents, including species whose appearance had never been explained by the two-partner model. The finding did not overturn the partnership account so much as complicate it: the alga still supplies sugars, the primary fungus still supplies structure, but the third partner appears to shape the form the lichen takes. What is the main rhetorical purpose of the final sentence?