Is AES really used for Top Secret stuff? I've read almost everywhere that AES-256 can be used for Top Secret material (in the US). Is it really used or is it some kind of decoy to hide the more advanced algorithm they might use ?
Decoy for whom?

Security, and cryptography specifically, has a need for public scrutiny. It's been proven time and time again that hiding the nature of the protocol/algorithm/scheme doesn't provide any tangible security. Kerckhoffs' principle explains this:  A cryptosystem should be secure even if everything about the system, except the key, is public knowledge.

It's "secure" up until someone finds out how it works and breaks it. Publically reviewed algorithms are something that people are always trying to break. The more people that try (and fail) to break something, the better that something is. This is important because it's really difficult to design crypto properly. 

To answer your question specifically: Yes, AES is used for classified materials. Other algorithms are also used depending on the nature and use of the data.

A quick Google search came back with this PDF doc from NIST: Guideline for Implementing Cryptography In the Federal Government.  It may be out of date, or there may be better documents, but it's an enlightening read.