He's been making guarantees he can't fulfill regarding health care for weeks now. In this case I don't think President Obama is lying; I think he accidentally spoke a truth AARP wanted to keep quiet. AARP isn't acting in the interest of its members; it's putting the Democratic Party first.
On this one issue? I think many AARP members would like to know the answer to that question. If I had to guess, it would be because AARP is taking a longer-term and larger view. Democrats have been more expansive for senior support than have Republicans. If AARP can help the Democrats on this important vote, then they'll likely get more benefits for their membership. However, I have zero proof of this theory. But there is no way that this legislation can be construed as anything but bad for people over 55. The cost savings the Government will try to achieve by shrinking Medicare outlays proves it prima facie.