I disagree; I would say the feat trumps the hybrid description as a more specific rule.
AIP allows you to use a different implement with your class. For a hybrid character, both classes are 'your class.' This -to me- would mean the new implement becomes a new implement which is usable by both classes.
Alternatively, AIP allows you to use a different implement with your class; this turns whatever the new implement is into an implement which can be used by your arcane class. As such, it now becomes an implement of your class; the hybrid rules then kick in and allow you to use it with your other class as well.
edit: Consider how the monk would work as an example. Monk implements can be any weapon a monk is proficient with. So, let's say the monk takes a feat to pick up a new weapon; for sake of example, let's say Spiked Chain. The Spiked Chain now turns into a monk implement; a monk/paladin could use a spiked chain as a holy symbol.