Maybe because they can drink it faster. Glycmic index's and the equivalent index's around insulin response are literally the gold standard for deciding what's okay to eat. "Vibes" or "My opinions" do not matter. OJ is marginally "healthier", significantly because the fiber in the drink (more pulp the better) slows down (slightly) the insulin and glycemic responses.

Source: Family of diabetics who have actually lost limb to the poison that is refined sugar.

I have no idea if this might be related, but citric acid is known to speed up mixed absorption of non-protein sources by inhibiting proteolytic enzymes in the digestive tract (it is even used in some medicines that require faster absorption because of this).

To that effect, perhaps the glucose in OJ is absorbed faster than the glucose in other SSBs specifically when consumed with other foods, leading to this anecdotal understanding that OJ is "worse" than other SSBs among diabetics?