They’re unsure whether it’s from the planet or the star. Can’t they just check whether it’s from an ion of water or the stable molecule? (Which IIRC is a question of the spectra observed.) I remember from undergrad the chemistry in stars would often have pretty extreme ionizations like OH^-12. If it’s a high ionization and still on the planet that seems unlikely to be habitable as we know it (or something fleeting from a lightning strike).