Yes, that is true. You have to complete the CCNA:Security concentration before you can start taking the CCSP exams. Supposedly, there is content overlap between CCNA:Security and ISCW for CCNP. I have thought about that as well and have decided to do CCNP/CCDP before CCSP.
Im in the same position, ive recently completed my ccna now ive started doing the CCNA:Security concentration exam before i go for CCNP. From what ive read i think you have to pass the CCNA:Security before you can start the CCSP.