Note the quote from a while ago:

> They are mounted to motorized arms that will retract almost all of their surface behind the Thermal Protection System – the heat shield – when the spacecraft is close to the Sun.

Yes, there's a moving part. It only has to move twice (deploy, mostly retract) but it's still a moving part.

A motor that predictably moves an object from one point to another can be much more reliable than an array of heat-driven reciprocating engines that have to be in constant motion and rely on conducting heat toward the internals of the spacecraft in order to function.