I hate to be the bearer of bad news, but attempts to demonstrate this is a board defect are not likely to be successful, even with the knowledge of where liquid contact stickers are since this is almost certainly corrosion damage, which is from liquid contact. The little green bit of “fuzz” that’s present on the components near the chips is oxidation from electricity running through a component which is touching liquid. So I would agree with Apple on this one.
For the record. Liquid damage stickers on this model are of the UV variety and are best seen under UV light. They just look like little black circles otherwise. But they are less definitive in demonstrating liquid contact, or lack thereof than the sort of issue in your picture. (although there are two on the other side of the board from this near the edge). This is also an unfortunate place to get liquid damage since those chips are important in getting the machine to power up. They are actually USB controllers, but they play an important role in powering the board.
This is almost certainly repairable by someone who has experience microsoldering, in fact repairing that area of the board is fairly common, since it’s along the edge and tends to cause no power issues when it is subject to damage. So keep in my Apple is not your only option if they tell you it’s going to be an ugly price to fix it. But I would not expect warranty coverage. And Apple will likely also not be able to get data if your board isn’t powering on.