I am aware of the existence of ranks among the owners of black belts, yes. It is more surprising that they all seem to have black belts.
I think it is just the format of the comic, one of the characters legit started less than a year.
That being said, based on some google searching, judo in Japan is taken really seriously, which makes sense.
There are levels of black belts of course, but there is a pretty hefty group of practitioners, organizations, and school programs to develop talent for judoka at a pretty early age.