assumign this is arguing in good faith..
the issue is not about it being american as it is america being in control of it. you don't get access to windows or mac os source code. You can however take the linux source code, fork it and make it yours. that "dictator for life" in portland can't stop you. nor can anyone else in the us government for that matter.
But technically you can also do that with chromium and gecko, but it's a lot of work, so very few do. And those that do don't cut the line, they'll almost all still follow upstream and just apply their changes.
So in the end, they're still dependent on the decisions made in the US. That doesn't need to be a problem, but I don't think "you can get the source code" really changes that.
Not to mention that many of the most important open source events and organizations are based in Europe.