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

[ShinryuReishiki]

By comparison, neither DRG nor SMN were in the top 5 groups for T13 (NIN was, though). It became a serious issue with FCoB, not before.

World first T13 was on November 2, and the same group cleared it with SMN on November 4, day of the first weekly reset. That's still two days before the world third clear on November 6. It's also worth noting that world first T10 and T11, and world second T12 were with a SMN as well.