I'm sure he said some of these things, but I doubt he was intending to be cruel and I suspect people are taking it out of context. And some other people just have an axe to grind and will twist a good natured joke into being a verbal attack.
Here's one report from someone there.
https://twitter.com/CultBoxTV
"Overriding memory of @DWFestival today: Steven Moffat being unforgivably cruel to an autistic guy who asked a question in Q&A."
"If Moffat genuinely couldn't hear the poor guy, I sincerely hope he's tracked him down and apologised since. It was horrible to watch."
"Even after he'd explained that he was autistic when a comment about liking Series 8 because it felt like the RTD era was misinterpreted.."
"...as criticising Matt Smith, he continued to lay into the fan in front of a packed audience of people. Genuinely disgusted to be honest."
"It was evident (to anyone with any sense or compassion) almost as soon as he took the mike, literally shaking, that he was on the spectrum."
You know, it's not exactly a State Secret that Moffat is a bully (just ask Caroline Skinner, oh wait, you can't) but given that he's being paid good money to turn up at a Convemtion and interact with fans, many of whom are children, you think he'd make some effort.
It is worth remembering, though, that this is one persons account of it. We all know how unreliable social media can be. But we don't know how this tweeter already felt about Moffat, what his agenda is etc.
He could of course he telling the truth. On the other hand, he could be one of those people who'd watch him walk on water and then post 'Steven Moffat can't swim!'