I'm not a lawyer, but I do at least understand the basics. A motion for summary judgement is a pleading that basically argues that there are no material facts in dispute. The only question is whether that set of undisputed facts supports a judgement for one of the parties, in this case, the plaintiff. Fox appears to be screwed. They are now making claims about context, but those are likely to fall on deaf ears. Summary judgement is hard to win, and it probably won't happen here, but Dominion filed because they believe there is no issue to be decided. They believe their case is that strong.
Suggesting they are doing this because they know they have no case is so absolutely dumb that it hardly deserves a response. If that was the case, Dominion would simply drop the suit. Good grief, is it really that hard to understand? I guess so.
That's exactly what summary judgment is, good explanation. You're also right that no plaintiff would file the motion if they weren't absolutely sure that the case is going to go their way.