Fantasy fiction can certainly be a means of escape. Leave the stresses and unpleasantness of the real world to get immersed in a book (Harry Potter, say) about a world filled with fascinating things you can’t access in reality, and where characters face greater hardships but overcome them in a compact time frame. Or a video game where you can be powerful enough to slay dragons. Or even an animation about cute magical ponies that manages to be positive enough to cheer you up every time without being sugarcoated enough to be annoying.
There’s nothing wrong with this, by the way, unless you take it to some ridiculous extreme.
It doesn’t have to be an escape, though. Escapism in the sense of an escape from something implies there’s something you want to escape from. What if there’s just somewhere you want to go to? There doesn’t need to be anything wrong with this world for you to want to go somewhere else, if that somewhere else is awesome. Fantasy arouses my imagination in ways that have nothing to do with anything but itself. Just because I want to ride on the back of an armoured bear, travel in the TARDIS, or run away from things with Rincewind, doesn’t mean I don’t also want to be here, where I a really am. In fact, I’ve found in recent years I don’t have time to read as much fiction as I’d like because I’m freely choosing to read so much nonfiction. I still like the fiction and would definitely read more of it if that were possible.
Talking about “escapism” in fantasy may be missing the point. It can be more like “voyageism” — and even when it really is escapism, that side is also present.
(PS. If you look at the Wikipedia page, you’ll find it also details a theory where there are two kinds of escapism, and “escapism in the form of self-expression” does rather sound like what I’m describing. Thus, the “voyageism” might be classified as “escapism” too, I’m not sure — but either way, it’s still not the “escaping something” kind of escapism.)