I am going to pick up from Teproc here. Granted, none of those events have happened ; but neither have those of your random convoluted action movie. We understand the things that happen in Gravity in the same way we understand physics on Earth, regardless of whether they have actually happened. If you made a movie about a hurricane in Germany would you call that science fiction? It had never happened, but there is nothing much mysterious about hurricanes or Germany - except for, you know, the Germans. The chain reaction would also be guess work, but so is your average action sequence involving large vehicles and collisions.
It seems to me that your definition of the genre is based on its tropes from way back when. Everything involving space in the fifties was definitely fiction, and certainly most time travel stories involve science and machinery. As science evolves however, surely the boundaries of the genre must too change. Jules Vernes' 20,000 Leagues was science fiction but I don't think you would argue Hunt for the Red October is too.
To go back to About Time, yes, as I said, time travel is indeed typically associated with sci-fi. Those movies always present one with some explanation for why time travel is possible or how it works, from silly Deloreans to big energy machines (Terminator) or black holes. The whole point of science fiction is to imagine the possibilities of discovering new technologies, or in some cases, living in universes with slightly different physical realities so that things like time travel or travelling at the speed or light are possible. About Time doesn't bother with that and just gives its characters a magical ability.
Contending that time travel belongs to the realm of science fiction in any and all cases seems like a historical argument that does not allow for new precedent. It's always been in this genre so from now on it shall always be. Would a movie about a boy visiting alternate dimensions with a magical trinket belong to sci-fi too? Alternate dimensions are an old trope of the genre too. What about a story about thinking machines forged by the dwarves, or exploring the cosmos with a couple of interstellar demi gods and their space caravel?
I haven't seen About Time, but I think it's very possible for something to be both fantasy and sci-fi. See also: Star Wars.
As far as I am concerned, SW is more fantasy than sci-fi. Not only is there straight up magic in it, Lucas completely disregards all laws of physics, biology and reason in his worldbuilding. Clearly evolution does not exist in the SW universe, the speed of light is something to be multiplied, there is sound in space, you can create a laser beam that stops at a given length, an inconceivable number of planets supports life, not to mention every single thing surrounding robotics and artificial intelligence. His is a realm that utterly belongs to fantasy but has been dressed with the trappings of space stories.