Neither Obama nor his administration directly and explicitly "promised" that unemployment would top out at 8% if the stimulus money was approved. However, they did strongly suggest it, and used that as their main selling point as to why the American public should support the massive expenditure.
Now, in hindsight, I can only come up with two possible scenarios that make sense.
Scenario 1, the administration and their economic advisors are supremely incompetent and had (have?) no clue what they were (are?) doing. Scenario 2, they knew what they were doing but sold the American public a bill of goods to be able to waste hundreds of billions. I guess a combination of #1 and #2 is possible as well.