The Tea Party could never put forth a "credible" Candidate because no matter what candidate the Tea Party puts forth, they will be human and flawed. Because they exist in real life. And because of that the media, as well as the author of the article, will see these flaws and immediately cariature that candidate so that they become "uncredible" in their eyes.
In other words, they are begging the question.
Presidential politics is about getting the middle of the road voters. The TEA Party (I recently learned TEA is an acronym or at least was) isn't built to get those voters; it's a Rove-ian 50.1% strategy that they embraced.