They put the warning out a day earlier so they didn't know it would be a failure at that time. The claim today was meant more as a warning that despite the failure they are still there.
They pre-recorded the message and posted it before the bomb was triggered. Pretty much proof that they did it. They didn't expect the bomb to fail. You are about the 500th person to ask that question between the 4 or so articles I have seen posted on the subject today, I guess none of you can actually read articles, only the headlines.
Now let's suppose they claimed it after the fact, the way you thought they did, they would still claim it as a warning that they are here and could do us damage.
The fact is, they are responsible and now the MSM will have to eat their frickin' theory that "white right wingers" set the bomb.
why would they claim a failure...???