There are probably very few parts of the code of a pacemaker that requires knowledge about medical devices or pacemakers specifically - probably the drivers tha

There are probably very few parts of the code of a pacemaker that requires knowledge about medical devices or pacemakers specifically - probably the drivers that connect to the sensors and the algorithms that determine what the pacemaker should do based on the result of the sensors. The majority of the code, especially if as said in the article it has wifi and listens to external requests, is code that requires only expertise in more general software, of which there are many many people who are qualified to review such software (or write it).