The entry on AoN and in the Adventurer's Armory is kinda wonky, luckily it also mentions another source that they appear in.
If we look at the entry in the Pathfinder Chronicles: Campaign Setting (written for 3.5), we can see the following entry for them:
Thistle Arrow: These arrows are a specialty of the Ekujae shamans, who craft the arrowheads out of the thistles of a toxic plant that most creatures find highly caustic. They deal normal damage but then become embedded in the wound and deal an additional 1 point of damage each round for 1d6 rounds from their irritating sap. Creatures immune to poison are immune to this extra damage. A creature can remove an embedded thistle arrow as a move action without provoking attacks of opportunity, but doing so deals an additional 1d3 points of damage as the thorny barbs are pulled free. A DC 12 Heal check (made as a standard action) can pull free a thistle arrow’s head without dealing any additional damage. A single thistle arrow costs 1 gp.
Given this entry, I'd say (in Pathfinder), they're supposed to deal the normal damage for a hit, and then deal 1 point of bleed damage for 1d6 rounds.
This logic is even more supported by the heading of the section in Adventurer's Armory for this item:
This book updates several items from the Pathfinder Chronicles Campaign Setting to the Pathfinder RPG rules.
Given its location near the end of the page, this would explain the funky wording and lack of an explanation given for how they work.
As they're currently written though, they wouldn't deal any damage on impact, and only deal the 1d8 bleed for 1d6 rounds.