So, I apologize if I gave the wrong impression. I try to use words carefully, and sometimes I am ... very careful. Do I think it would be possible to plead a plausible defamation claim that could survive a motion to dismiss based on what is there? Yes.
Do I think that this complaint has that? Not really.
Am I going to explain it further? This is the internet, and given anyone can read what I write, I would not want someone to read an idea that I have and use it to prolong litigation in case I might offer something helpful.
Does it matter? Even assuming a plausible (not a winning, but a plausible) defamation claim is pled, the issue that immediately comes up is ... where is the causation with damages (proximate cause)? The statement was up for less than a day. It's probably going to get a lot more publicity (Streisand Effect) from the lawsuit than it ever did from being briefly posted on facebook - I mean, I try to keep up with the TTRPG hobby and had never heard anything about it. There's the weird use of passive voice in a key allegation ("Due to Luke’s post and how his dishonesty negatively impacted Chris Clark’s reputation and put into question the integrity of the third installment of Castle Wolfmoon, it was decided to postpone the launch of the Kickstarter.")
Really? It was decided? When? Who decided? Why, exactly? Was there ever a really firm launch date? What are the real damages?
If I had to peak behind the scenes based on the allegations, I would guess that the real issue is pretty simple. The Gygax family didn't want Gary's name listed first, since he wasn't the "author" (Chris was). And ... I think we all know that having Gary's name as the first one probably was important for marketing if you know what I mean.
But instead of reasonable minds coming to a solution, we have this. Like I said, sad.