The thick potion does not effect items held items or items in the hotbar (regardless of the thick potion's location)
This is a good example of the mod in general. Though it is a shot from the 1.4 .The two drowned are holding different light sources. (and i am holding a crying obsidian one... the torch doesn't work as i'm underwater)
The thick potion does nothing when in the hotbar.
in creative if you hold on to an awkward potion for 15 seconds you will be told you are now in hard lighting mode where only held light sources generate light.
in creative if you hold on to an mundane potion for 15 seconds you are told you are normal lighting mode (the default mode) where light sources work from anywhere in inventory (or the hotbar with a thickpotion).
When the thick potion is the inventory light no item in the inventory will produce light.