Prior to the Texture Update, the bottom texture of the enchantment table had a faint ring around the edges, making it visually discernible from obsidian. However, the current texture is completely indistinguishable, meaning that there are no differences at all or that any existent differences are insufficient.
The difference still exists in Programmer Art.
Confirmed/Reproduced in 20w29a.
The new Enchanting Table doesn't have borders in general, so adding a border only to the bottom wouln't look so good. The sides would have to be changed too, but there probably is a reason they didn't.