Unless/until there is a risk that the chips themselves are backdoored and trying to exfiltrate data, European companies that host in Europe still solve a big problem for use of certain data in Europe.

It's not a purity test. Relying on US chips in not the same deal-breaker for all but the most extreme situation as relying on a poorly regulated US company to run the inference.

Out of curiosity, how does a chip that does inference calculations exfiltrate data without being seen?

Has this happened already or is it just conceptually possible?

Sure, but it's probably not that easy to exfiltrate multi-terabyte datasets without being detected.