On that page you linked, they say “So far, the EU’s reach has not been tested, but no doubt data protection authorities are exploring their options on a case-by-case basis.” So it hasn’t really been tested yet it seems. It’s true that there are extradition treaties and interpol that aid in cross-border prosecution, but that tends to be used primarily when the alleged crime happened in the prosecuting country’s jurisdiction, or the alleged crime is handled similarly in both countries. A GDPR violation by a US company wouldn’t be considered a crime at all in the US, so it’s entirely possible that they might decline to assist in prosecution.