This pops up a lot, and I'm always wary when I see it. The short truth is no, a disclaimer doesn't create a legal shield. You can write 'I don't own these characters' at the top of your 'Harry Potter' fanfic until you're blue in the face, but if the copyright holder decides to issue a takedown, that text means nothing in a legal sense. It's a community norm, a sign of respect, not a get-out-of-jail-free card. The safety isn't about the disclaimer; it's about the copyright holder's tolerance and the platform's policies. Wattpad itself has a huge amount of fanfiction, and most rights holders look the other way because it's free promotion and builds their fanbase. But that's a choice, not a right granted by the disclaimer.
Where people get into murky water is thinking the disclaimer protects them when they're doing things that might actually cross a line, like posting entire chapters from a published novel as 'inspiration' or writing extremely explicit content for a franchise aimed at kids. In those cases, the platform might act even without the copyright holder stepping in, because it violates their terms of service. I've seen stories get taken down for that. The only somewhat 'safe' reuse under copyright law is for parody or certain types of criticism, but that's a high bar to clear and most fanfiction doesn't qualify. Honestly, the system works on a don't-ask-don't-tell basis fueled by fandom goodwill.
My rule of thumb is to treat disclaimers like a courtesy nod, not armor. If you're posting fanfic, you're already operating in a gray area everyone accepts. Just don't monetize it, don't claim it as original, and be prepared for the small, small chance it might vanish one day. That's the real ecosystem we're in.