I'm Miranti, the owner of the recipe for Pad See Ew in question (http://highheelgourmet.com/2013/04/18/pad-see-ew/) ...Hello everyone.
The sweet dark soy sauce isn't the "particular" preference for the dish. I used it because I don't want to use sugar. You can use dark soy sauce and add sugar too, as Puri mention in his comment. This dish shouldn't start off sweet. This dish or any Thai dish shouldn't start off sweet. Sweetness is just to "round up" the flavor but not the lead. We're not making dessert here.
When I cook stir fry noodles or stir fry anything that I need dark soy sauce to add color to the dish, I would pick sweet dark soy sauce instead of using dark soy sauce with sugar. The magic ingredient in the sweet dark soy sauce is the "molasses". The most sweet dark soy sauce would, in general, have higher molasses content than dark soy sauce.
The taste and flavor of molasses can add more dimension to the sweetness, not just bland one dimension like sugar. It helps if you cook with the household wok that might have non-stick coating or didn't have deep seasoning flavor in the wok like the restaurant's woks that has been through cooking over hight heat several thousands times and packed those "wok well season smell" on the surface. When you cook the noodle in those woks, the noodles would pick up the yummy smell and that completed the dish for me. (The flavor of an experience street chef's dish...even the replica is better than none). In order to mimic that smell, I used sweet dark soy sauce with high molasses content and omitted the sugar just because I can't have the restaurant wok! I also doing the same thing with my "Drunken Noodles" (http://highheelgourmet.com/2013/07/27/drunken-noodles/) too.
Another note about the brand, I use healthy boy brand, dragon fly brand, golden label (least favorite due to high MSG content), and a few others ALTERNATELY. I'm suspicious that there might be some kind of additive that quite foreign to our bodies or at least my body and I don't want to give it a chance to let them collect in my system. So, I keep switching the brand as soon as I finish one bottle (sound paranoid but I rather be paranoid now than later). I can adjust the taste because I taste test everything but I can't adjust anything when there is lump or tumor appear in my body.
My favorite brand is dragon fly but that also don't make me buy it more often than the other brands.
Sorry I should have seen this earlier but I was busy and just saw the link.
Thanks for explanation about the healthy boy brand. I would keep the link in case I need to refer to the different type of the soy sauce!