Where were the other guilds? You gottta kill Vulak and they beat them to it. On Phinny, when OGC kill AHR first, they won Luclin. They lost almost every open world boss in that expansion, but they won the expansion. That's the game.
IMO there are some expansions where it isn't that simple.
AHR is beating Luclin, yeah. It's the concrete endboss and has no competition. Someone might bark about Khati Sha being harder or something but nobody counts that or cares.
However for Velious, it's more convoluted. Vulak is considered the end boss, but for some guilds AOW is the harder fight that takes more preparation. Defeating the Warders is the thing that requires keying up and actual strategy as a guild (splits, prioritizing, etc) rather than just bumrushing a boss. So I would argue that in Velious, the first guild to clear Vulak, AOW, and Ventani the Warder (assuming nothing has changed and he's still the hardest by far) is the winner.
Later on this comes up again, specifically in The Serpent's Spine which has two alternate zone paths / progression paths / raid paths. Beltron and Lethar are both equivalent final bosses, and the first guild to clear both is regarded as the winner. IIRC OGC and AOS went different routes and both cleared one of the two final bosses before the other, then AOS was quicker to get the second final boss down.
At the end of the day, Classic, Velious, and TSS are the eras that I would say have multiple bosses as their endzone rather than just a single final boss. The end goal needs to be established before Velious, otherwise a guild can declare victory after the fact based on whatever criteria matches what they were able to do first.