Can someone give me an in-depth explanation why Summoner is arguably the worst DPS?

[Viridiana]

A Dragoon who is good at their class could clear T9 fast, so what?

A Good SMN in a top-end team could have been world first for T13, it would not change a thing.

I doubt it? I mean, maybe they could have replaced the BLM, but I doubt they could have been world first at that point. I mean, they didn't even make it into the top 5 groups.

Edit:

The important part for those teams, would have been learning the mechanics first, for players of that level, the dps would not be a major issue once they knew the fight.

Seriously? World first was literally as Gigaflare was going off. DPS matters plenty at that point.