Probably a China brain fart. Ruby is extremely hard. Just below diamond. On a regular dial indicator running continuously on a surface, there is much less wear on a ruby than on a hardened steel tip. No other advantage that I can think of and probably not attributable to a probe which doesn't see much wear (that I know of) like an indicator would.
Also entirely possible that it isn't really a probe tip, it just happens to fit and was actually made for indicators.