The Fictionary defines direct damage as:
Direct damage – health loss caused immediately when a special is cast. Contrast to DoT.
Although the fictionary is user-created, it is maintained and curated by the moderators.
There is, of course, another way to figure out what SGG defines as “direct damage”—what damage can rogues evade? Because rogues can evade shared damage, it follows directly that such damage is direct. QED
Functionally, it’s also clear. Direct damage happens immediately upon the cast; no status ailment is needed to keep track of the damage.
I suspect what you are taking issue with is the (mis)use of the term “direct” instead of, say, “upfront” or “immediate”. I also dislike the term, but for a different reason: “direct damage” is actually filtered through the Damage Calculation formula, so it’s a calculated value, not a direct “inflict 300 damage” sort of thing.