No the two cases are different. If you want to do BTech in IIT then you have to pass you class 10+2 exam with 75% marks and by qualifying JEE MAINS and ADVANCED.

Now if you want to do diploma and also want to go to IIT then you can do Diploma then BTECH by Lateral Entry examination. After that you can give GATE and if you qualify with good rank you may get into IIT for MTECH.

Basically Diploma teaches you basic working principles of Engineering. The course isn't as deep as BTECH. In 10+2 you are given the knowledge of science which you can utilize later. So this are the very rough difference between Diploma and 10+2.

I hope my answer helps. All the very best for your future endeavors!