It's true, though. As someone who loves to give away days to a deep dive into fic, you need a well-rounded diet. There are brilliant fic writers out there, both talented hobbyists and award-winning bestselling professionals indulging in some fun, but as stated above, we're all using pre-fab characters/worlds/plots to fiddle around with ideas that interest us. Original fiction draws inspiration from all that comes before it, but still attempts to create something new. It's all the retellings and reboots vs. a new movie.
More than that, even though people like to say "I read fic that's better than anything published!" that's... not widely true. Sometimes it is! But fic is a thing that can go up without editing, without any kind of checks. It can be bad and ungrammatical and typo-riddled and nonsensical, and that's okay! Because fic doesn't have to be in any way good! It's for pure experimentation and fun for the writer.
Traditionally published works, however, are meant to make money. They have multiple sets of trained eyes that try to make the final result the best it can be. Sometimes the final work falls short of that goal, sure, but there are steps to at least try to filter out some of the worst elements, which means you're ingesting and internalizing fewer bad habits, which is crucial when you're trying to figure out how to do this writing thing.
To reiterate, reading actual books means:
You're more likely to have solid examples to internalize during your own learning process.
You have greater scope to read original work that at least attempts to do new things you've never seen before.
You're reading works that have been vetted and refined multiple times by multiple people who do this for a living.
You're reading things made with the GOAL of being polished, deliberately crafted, enjoyable experiences (as opposed to fic, which can be dashed off and is for the writer, not the reader)
Anecdotally, reading a really good book has always made me feel creatively sated to the point that I then feel like I need to release some pressure via writing.
TL;DR: Read widely. Read voraciously. Writers do not live on fic alone.
