You think that was a stupid mistake? Probably a deliberate documented procedure: throw the person off-balance with some unexpected "stupid" weirdness.
"You told the border guard you're entering the country to work." "What? No! I never said that! I'm entering to marry my fiancé!"
"I'm here because of the MBA conference." "NBA? We didn't know there was a basketball event in this building."
Yeah, I've seen it a number of times.
They weren't upset. They were manipulating the author. They weren't stupid. They already had agents heading to the apartment to investigate.
And throw in the fact that lying to a Federal agent can lead to felony charges, the behavior of his interrogators is very typical.
My favourite was a CBP asking me "what do you do for work" (lovely unspecific question, that) then pretending to get upset with me for not telling him I was unemployed at the moment. I resisted telling him I personally don't consider unemployment a career...
They were hoping to use his failure to identify it as proof that he was an adversary.
When it turned out that he had identified it, the person who had hoped that now had egg on his face. (I wouldn't be surprised if in another room they had figured this out as proof so that they could get him.)
If the guy is an adversary, you can't rely on him to bring that fact to your attention. If the guy isn't an adversary, his failure to identify it is (by definition) innocent in nature. In neither sense does harassing the guy about it make sense.