However even posting honest, accurate articles online can come back to haunt you. Unless written very precisely, words can be interpreted by different people differently and often in a way that wasn't intended by the author.
This process doesn't have to be a cynical twisting of words - naturally, a reader's interpretation will be informed by their pre-existing point of view. When that view is one of a litigant, it can open up new issues in a case and lead to new ways to analyse evidence.
If you are posting about a case, make sure you're doing it calmly and definitely get it edited by your lawyer!
[edited for accuracy, typos]
If he denied they were, he would be asked further questions and possibly caught in a lie. Such as "is this the only time that your account has been used by someone other than yourself?" or "So you are saying that this is the only tweet that you didn't make but the others before and after you did". And so on. My guess is that he would be advised to tell the truth to prevent getting further trapped as far as his credibility. There may also be other people that he discussed the tweets with that could be brought into the picture as well under oath in court I'm guessing. Bottom line: Denial is easier said than done.
Additionally I've noticed a loose correlation between people who lie and who their parents are or how they were raised. Nothing scientific of course, but people whose parents don't hold their feet to the flame are generally more likely to think that they can get away with something because "the other guy is stupid". People whose parents are either very intelligent or hold them on everything they say are generally more practiced at thinking of the various possibilities that can come about to refute something they would say.
Tweets aren't under oath or anything, but it opens up Carmack to lots of questions and possibly affects his credibility.
Often times cases come down to "he said / she said" types of claims and how a jury views them.
If ZeniMax mentions the tweets in court, it will likely be to try and make Carmack look like he acted out of hand, and that he rushed to make claims that arne't true (i.e. the IP / code distinction the poster makes).
Here's the thing about software suits. Most judges / juries are in a TERRIBLE position when it comes to evaluating the nuance involved in a suit like this. Think about the average person and how much they understand about what you do.
Lawyers will try to boil it down to themes and narratives that portray you as "bad." "Winning" in terms of how you evaluate claims or the press or your followers evaluate claims does not matter. It's all about how the lawyers convince a judge or jury you are wrong.