If you use the PPP adjusted per capita GDP as a comparison metric the US stands about 37% higher than the EU. Considering the baseline tariff of 10% and the substantially higher tariffs for important trading partners the US targets, it may actually even out.
But GDP (PPP) is not the same as purchasing power, no? I don't know of any actual data but you can imagine two countries, both with the same GDP (PPP), same population size, shared currency (so the PPP doesn't factor in). One has organized its production and labor force in such a way that everyone has a relatively equal share of the pie, and the other one consists of a massive wage slave class getting by on poverty wages and a privileged few who reap the benefits of production. Surely the first of these two countries is more interesting to global ecommerce companies from a sales perspective.
Does this include the fact that tariffs negatively affect even the EU GDP?
Tariffs in the US affect the whole world, not just US or EU - every single GDP goes down, the question is how much. EU gets hit less than the US is more or less a given, but error bars are significant.