I find it equally likely that the leve-plate itself was the catalyst or that we just aren't shown that a member of the party was actually at the pub at the time. It's clear that Adventurers' Guilds recycle leve-plates - they're pretty much like those lanyards you wear to get backstage at concerts. A lot of them look the same, multiple different crews might use the same lanyards at different shows... etc.
However, the Echo is clearly stated to resonate with a "soul" - so did one of the old party members RECENTLY handle the plate while having a vivid flashback? Had the soul essence, and the crystalized glass plate for that matter, survive 10 years of adventurers taking it into the field?
These are the main reasons I figure we have to either (A) pretend someone from the party was at the AG at the time, reminiscing about the decade anniversary or something, or (B) accept it as a lore-shattering inconsistency.
This one I don't think is likely; the guild is giving you permission to do normally off-limits things (giving you LEAVE, middle English "leve") to meet the CURRENT needs of someone who has used the guild to broker a contract between patron and adventurer. For instance, in Treasures of the Main (which is taking AGES to write a Loremonger for), Baderon basically gives you charity by giving you your first guildleve; he tells the guild to pay a sum of coin to whomever completes his contract, but then doesn't have them offer it to anyone and slips it in your bag when you're not looking. You go out, you solve his problem (tracking and eliminating a nest of rats that are raiding the Wench's stores) and he pays you. All in current time.
Also, there's this quote from the original FFXIV promotional material:
"When issuing tasks or quests, guilds will often provide adventurers with these plates, allowing their bearers "leave" to take whatever steps necessary to complete the jobs, including entry into normally restricted areas, hunting or harvesting on private lands, the confiscation of goods, even negotiations with those considered enemies of the city-states."
Sounds legit to me!