Yeah, I noticed that contradiction, too. Then after letting it stew for a bit, I think I've come to the conclusion that maybe its more a matter of being worded unfortunately rather than being contradictory. Basically, I think that the last quote was intended to set forth an exception for potions only to the rule about spending a mental stress to gain an extra use of an enchanted item. So in other words, yes, a potion can have multiple "servings", but once the potion is used up, it's done, and you can't use it again by spending mental stress.
Here's a simple re-wording that clarifies what I think the designers meant (ie, my take on RAI, not RAW):
A given potion slot can only be used"filled" (by converting it into a potion) once per session, period, but it doesn’t face a surcharge for being usable by someone else.