Reed Stalkers
Abnormal Seeds were dropped by all plant creatures, including Oakhearts which are pretty much outright confirmed to not be related.
It’s downright impossible for them to have been intended to be “early concepts of Sylvari” because, quite frankly, the idea of Elder Dragons didn’t exist until development of Eye of the North/Utopia. If they became such – which I doubt given the lack of them in GW2 – it would be a retcon.
Not all mobile plants must be tied to Mordremoth, just like not all undead are tied to Zhaitan and definitely not all ice elementals are tied to Jormag (in fact, none are).
They were just a humanoid mobile plant, really. In fact, if you look at their models they look nothing like how mordrem do – reed stalkers are spaced out reeds in a humanoid shape, while sylvari and mordrem are highly compressed leaves, vines, and other plant materials shaped around a wooden endoskeleton.
Stop treating GW2 as a single story. Each Season and expansion should be their own story.