By adding 1/16 shear webs at the front of the spars and giving instructions to only drill into them for the dowels I think the designer is telling us not to drill into the joiner. That would be picture 18 on page 21. This would be a good place for some of that really hard wood they supplied. A sixteenth of that hard wood probably equals 3/8 of regualr balsa.

Those dowels derive their working strength from the front dowel support which is why I pointed out the necissity of getting the retract servo plate in correctly. The servo plate is installed whether you use retracts or not because of its' use in anchoring the dowels.