This is pretty neat. Though it seems a little odd that a query like:

  INSERT INTO salary_grid VALUES (1, 'junior developper', '[20000, 30000]');

would produce a record like:

  1 | junior developper | [20000,30001)

Is it just an error in the post that a query with an inclusive upper bound would result in a record with an exclusive upper bound?

but this is exactly the same range. (note that theses are ranges in the ring of integers)