It might be okay. There's a lot of vegetation in the Amazon, so perhaps from time to time they munch on some fruit drops. And it's plausible that the insects they consume may have a gut full of fruit.
Not excellent for them if fruit makes up the majority of their diet, but it shouldn't hurt them.
A pair of long tweezers or a long skewer might let you get the fruit down to the pacu before the aros can get at it.
It may not be something they eat naturally, but it will have no bad effects. It may not process it fully but it won't cause any issues, if anything it'll just be some more nutrients in its diet.
Arowana would try to eat socks if you threw them in the tank. They're surface-feeders with big mouths meant for swallowing anything even remotely edible.