It seems unlikely that the GoPros in question are rad-hard (there is no rad-hard GoPro that I'm aware of); it's more likely that they chose them for some other reason (perhaps some sort of validation or testing), or that these have fewer heat dissipation issues than the newer models. Heat dissipation is a frequent cause of issues for electronics in space.
They are old because the design was locked long before launch.
If they found new cameras with better heat dissipation, they would still go with old ones, otherwise they would have to recertify the new ones, which would take ages.
