It's important to note that autism is a spectrum. In other words, not everyone with autism has the same ways it outs itself. Some have issues expressing themselves, some have issues understanding others, or some even have issues understanding the world around them.
Mostly, symptoms show at a young age, around 1 years. Depending on the severity, children can live a mostly normal life, especially when there has been a good therapy.
In general, children with autism follow a different learning pattern compared to other children, at their own pace. But normally, learning stays pretty linear, which means they don't lost skills they've already mastered.
So to answer you question, a child with autism that has learned how to talk, will normally not decrease in capabilities to do so. If they do, it often is a sign of another underlying condition.
Now when we talk about online, we must first note that typing is not the same as talking. We have different parts in our brain, which are responsible for talking, for hearing, and for reading and typing. So it's important to note that online communication does not link to the ability of someone to speak in real life.
The story you told about 2013, could have had possible explinations as to why it happened. Maybe they didn't know the show, maybe they were too young to express themselves, ...
Just be sure to know that a negative experience in life, both online or offline, does not indicate an issue with yourself.
I hope I was a bit clear with my explination, and I was able to answer the questions you've had. If you have any other questions, be sure to let them know!
Edit: As Azoreseuropa metioned above me: Autism does not necessarley affect intelligence. In fact, some children with autism turn out to be well above average.