@CuddleStruggle they needed some way to beat the time stop dragons, otherwise they would instantly lose, that’s why the writer gave loopholes to Artimesia’s powers as well, if you give an antagonist such a powerful ability like stopping time, the protagonist needs some way of getting around it, but for the sake of the story, it would have to take them a bit to figure it out. TLDR: it’s so the author doesn’t have to invent a completely bullshit power up for the protagonist so they can beat the DIO wannabe.
Also the light could’ve already filled the room in the split second the light grenade went off in real time, already filling the room with sunlight, killing the time stop dragon in the stopped time as it was basically surrounded by acid