If you follow the links back, the story was originally told by KFOR at http://kfor.com/2014/10/04/oklahoma-muslims-receive-special-praise-from-white-house-officials/ . That article has a quote from the letter which was read by a regime DHS guy and mentions what the imam said in the sermon the day that the letter was read. So it seems to be an eyewitness account.
Thank you