Good interpretation! I tend to be a little generous in allowing cover. Well, the +2 version, at least.Reading the door as a full 5 feet wide opening, I would say the slaad has no cover to the rogue, as the rogue has full view of one of the slaad's squares. See the DMG quote and illustration below. If the door is only as wide as shown on the map, then yes, half cover would apply.
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Cheers,
Merric