In practice, yes, as you point out: US firms must have assets in Europe to compete effectively.
But even if they didn't, Europe could deny access to the European market. So there is no reason to try and minimize surface in Europe. e.g. Apple has to comply with European antitrust rulings about app store access, even if Apple were to just sell their product to third party distributors in Europe and not have any presence in Europe.
Not really, Apple would just get customers the same way they’re sold in any other part of the world that doesn’t officially have iPhones (i.e. Russia), the EU doesn’t have the authority to seize shipments purely based on a violation of the DMA.