Right-- the group of students who qualify under IDEA (for an IEP) is wholly drawn from those who WOULD otherwise be qualified under Section 504 anyway.
Ergo, while an IEP doesn't follow the student to college or into the workplace, the protections afforded that person under ADA most certainly DO/CAN follow them.
So don't worry about 504 provisions being under the aegis of the IEP. That's as it should be. For now.
If the student ages out of IEP eligibility, s/he should still be eligible under Section 504, and then that portion of the previous IEP will stand on its own as the provisions of a 504 plan.
Same thing in college.