[smrk007]

But they're calling it a third sex because it's due to "normal expression of the species' genes", not because of hermaphroditism.

[empthought]

It is a third (or fourth) phenotype, not a third sex. The abstract of the source article is careful to spell this out; the clickbait site introduced the error.

"Three sex phenotypes in a haploid algal species give insights into the evolutionary transition to a self‐compatible mating system." -- i.e. how might self-pollinating plants have developed?