They would have more of a case for this ad if all default units were mono-color as warchest skins are.
But as it stands, this is like false advertising. There is pay for advantage in SC2. If you buy skins, your units are mono-color and hence you have the advantage of making it harder for your opponents to distinguish your units (e.g. immortals are grey, stalkers are grey).
If you don't buy skins, you don't have this advantage, they are easier to distinguish due to different colors (e.g. immortals are yellow, stalkers are grey).
Sure, it's not as much of an advantage as buying heroes or power upgrades, which is what the ad was clearly making fun of, but it's still buying an advantage.