The mathematics which the students study now at the undergraduate level is different from what they study for the JEE preparation. There are only a few courses of applied math where you will do the coding, IISER Kolkata offers one such course.
Lastly, if you would like to pursue research in Math then definitely join IISER Kolkata, or if you would like to only do applied mathematics (coding) and obtain a job then you ought not to choose IISER Kolkata.