That's not exactly what was said, if you watch the actual TTS I can't think of any reason why he would run to the basement that's more likely than that the account is his and TJ knows what was on his account.
You can also see that he snaps his head midway through the TTS, only after it becomes clear what it's about, he does not have any reaction to "TJ".
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The fact that it didn't mention his username makes it seem even more likely to me that it's his account. If he got a TTS that said "we know you are this person and posted this on one of the accounts", it would make sense that he would want to go to the basement to convince production that that's not actually him even though he uses that username.
Right, it was December 31st if the "New Year Party tonight" is anything to go by.