Guild Wars 2

Title says it all… How is this possible?

According to Foldach "There is no memory of this place in the dream. The Pale Tree had heard of it, of course, but no sylvari had ever walked the decks and experienced it. "

So… a Sylvari got into the group and became a fully fledge member in 3 weeks?

It seems so. There is a conversation between a Zephyrite and a Sylvari, they are just next to Kiel, and the Zephyrite seems to be confused about the Sylvari “awakening” instead of being born.
That Sylvari probably just joined them. Like, last month. Literally.

Also, have a NPC that dialogue says “Everyone is welcome to join us, but need to follow us where we go” Or something like this.

So, its not any hard that even in this last 3 weeks, or before (In another landing) a sylvari has joined they.

Sylvari personal story has… a sylvari that was not born of the Pale Tree and had never even heard of it. Nor does he have any memory of the "dream", hinting that there is another Pale Tree like growth somewhere else in Tyria. Possible this other is from there.

Other than that, maybe he did just join when they arrived. Try using your imagination.