Earlier quoted context omitted.
They really aren't all that different from each other. One is imaginary things that might one day be possible, and the other is imaginary things that won't ever be possible. And even then, that can swap between the genres. Scifi often contains FTL tech, which from what we know is almost certainly impossible so it's actually more like fantastical magic. Meanwhile, fantasy can have hard rules for its magic, in which ca…
I never find it helpful when people say they aren't that different from each other. Sure there may be some similarities if you want to take an analytical view of the genres, but there's an awful lot of people who like one but not the other.
The problem is once you look at the definitions it's actually quite hard to exactly define what's Fantasy vs Sci-fi. It's more a venn diagram, than strictly separate genres and everyone has their own definition of which is which. So when someone likes one but not the other, it's hard to discuss books because what one person considers sci-fi, another may consider fantasy pretending to be sci-fi, thus the complaints of the original commenter.