To say "It's a miracle" implies that "it's amazing" or incredible.. or any other synonym. The writer is saying it's amazing that people can say it with a straight face.

Hello. In my opinion the sentence is correct. I would suggest adding a comma like this: The miracle is that, there are people who can say it with a straight face. The comma here permits the two sentences to be joined in one. The miracle is that in this case means, that it is surprising/unusual that there are individuals who can (whatever is being referred to in this case) with a straight face. Hope this helps.

It's normally impossible to say it with a straight face the miracle for him is some can say it with a straight face.