I don't see a reason why only plain water would cause hyponatremia, although practically speaking, plain water is probably the only low-sodium beverage that people would commonly guzzle in large enough volumes to cause dilutional hyponatremia. It's the effect on serum sodium levels that matters, not how you get there. There are multiple mechanisms that can lead to hyponatremia.

Guzzle enough decaf coffee and I see no reason why that could not also cause hyponatremia. Switch to the caffeinated version and then you will induce diuresis and could possibly trigger a sodium imbalance that way, too. Well, on a hot summer day, someone could conceivably guzzle a lot of plain iced tea in one sitting, too. That's another low-sodium beverage. After a long day of working in your yard, losing sodium in your sweat all day, that pitcher of iced tea you gulped down could certainly lead to dilutional hyponatremia. Likely? Not really, but I think it's certainly conceivable IMO.