Actually, I was wrong, I misunderstood somethings. Not afraid to admit I was wrong.
I just talked to the forensics guys I know, I was wrong. And you are correct.
That is partially correct, modern forensics tools (like FTK) can fight back.
http://www.accessdata.com/products/digital-forensics/mobile-phone-examiner On-the-Fly Decryption of Operating System and Logical Data of iOS
Without the development of a secure wipe tool for mobile devices, all your information is easily available to retrieve as long as you know what you are doing. Look up tools like FTK or Encase.