Discord still failed in the end, and the "MacGuffin" doesn't necessarily overpower you, it's like you said, it just solves everything. It could have just sucked the power out of him. Anyway, the show made it clear in this scene that Discord was powerless to stop Tirek, he couldn't just snap his fingers and win.
The fact is, Starlight manipulated time for her own selfish desires, attempted to harm and ruin the mane six's lives. That alone is a huge deal, not counting all of the brainwashing and manipulation she's done in the past, or the entire year she spent holding a grudge and plotting revenge.