I think their point was that J and SQL target different domains, and each will be stronger in its respective domain. If I want to join two tables, filtering on some value, sorting, and viewing the first ten results, SQL will likely be the cleaner syntax. If I want to apply a polynomial function to a list of values, and calculate the standard deviation of the result, J will be much cleaner.

There's some similarity, at a high level, with how they work. But it doesn't really make sense to say one is, overall, better than the other, because they don't solve the same problem. It's like saying a hammer is better than a screwdriver.