Not how I would have expressed this, but my wife said it is a normal expression. Her translation ... They speak well/good. And yes, she said mantap means steady. Again, different language, different colloquialisms. I have long accepted that I will never grasp these are there are far too many to learn.
Yes, however, Indonesians would say 'lancar' instead of 'mantap'. To us, the overall meaning is the same. Maybe, to Indonesians, there is a subtle difference.