You're right in that it doesn't have any effect.
HP restoring berries activate at 50% anyway, so Gluttony doesn't have any effect.
I think your mistake is thinking that people regularly run that set, I never seen one and I had a quick check on Smogon and none of the Pokemon that have Gluttony as an ability use Sitrus berry with it in their sets.
Are you sure you've seen this and if so are you sure it wasn't just a new player not understanding?