I've always had issues with that phrase, mostly because I'm a huge fan of sci-fi and have never been able to figure out how someone can 'know' about alien species or other planets or tech that doesn't exist.
I know, I know… it isn't meant to be taken literally. More like If you like sci-fi and read a lot of it, write sci-fi
I like sci-fi. I read a TON of it. If I read 200 books a year (not unheard of) then 100 of them are sci-fi related.
My point? Apparently nothing I actually write qualifies for sci-fi. Go figure ;)
(NOTE: this is said with tongue firmly planted in cheek. Do not take literally)