But Gaiman never says anything about commercial success. In fact I've never seen any author say anything like that. He says write, finish what you write, and do it a lot, and you'll be a writer. No mention of publishing. No mention of commercial success. He isn't saying you're not a writer if you're not published or a commercial success. He's basically saying the only way to learn is to do, and that practice makes perfect (or at least competent).
Conversely, if you say you're going to write, and don't write, then you're not a writer. Full disclosure - this is probably me!