The retraction post from Gruber details ways in which he tried to fact-check this, including confirming that his understanding was correct with Mr. Godier and even running a draft of the post by him to make sure he'd stated the "facts" that Godier presented him correctly.

I'm not really sure what you're implicitly expecting him to have done "better" here. Ask Godier for a signed affidavit? Not publish this until he could get a statement from Apple to get their side of the story? (Which is to say, not publish it at all because the chances of Apple giving anyone "their side" of something like this is essentially zero.)

I make a real fact-checking effort. In this case, the developer (Godier) sent me a video of the app in action. It’s a real app. He couldn’t send me a TestFlight to run (he said) because TestFlight submissions are taking a week or longer to get approved.

Obviously what I did wasn’t enough, but trust me, I try.