The description does not describe the venting into the tank but it must be a function of the cap to allow free air flow into the tank to displace the fuel being pumped out. What they seem to describe is only the relief of excess tank pressure at a threshold level and seal off fuel loss with a full tank sloshing at the cap. Thus, it has to be bidirectional.

I almost regret posting the manual page because it may confuse the issue more than answer it.