IRIG-B is simply the designation for the version of time code. There are also IRIG-A and IRIG-C through IRIG-H. IRIG Standard 200-98 describes the various codes. IRIG-A and IRIG-G have count intervals of 1 ms and 0.1 ms respectively.
The IRIG-B timecode itself indeed only has 10 ms resolution, but that's never been an issue, since all IRIG-B decoders are equipped with a crystal-based real-time clock that's synchronized to the time code. This allows the time code receiver to have usually better than 1 us time resolution under all conditions.
TTFN