True, but big horse power implies big heating. Lots of cylinders and multiple rows, high alts(therefore v.cold temps) and higher speeds does seem like a recipe for problems.
RPM isn't really relevant in this, rather the specific output of the engine and the relative thermal inefficiency of all piston engines shows that the bigger and more powerful the engine, the larger the heating effect.
Combining the two things (high engine temps and a colder ambient temp due to the issues above) I can understand that radials will suffer the problem of shock cooling but I don't think that the relatively tiny lycomings and conti's will suffer compared to the different animal that the radial is due to their different design and utilisation.