Your opponent has to shield PREEMPTIVELY against Zelda Up-B. That is not a trivial commitment. This makes Zelda very difficult to pin down. Keep in mind that Zelda has MANY options to mix up her mobility both when factoring the direction she can travel, and the way her momentum is altered with her midair jump while Up-Bing.
Keep in mind also that Zelda's footsies are in fact good. She doesn't just sit back and watch you run at her. She has strong moves in midrange like Jab, D-tilt, Dashgrab, and F-smash, with an occasional Down-B and Side-B to keep you honest. And when she gets her hits in, she has several options to trap you at the edge or in the air for reliable KOs.
That Zelda player from Japan in general has several GOOD examples of how Up-B forces people to make commitments that Zelda either can use as an opportunity to get out of jail free, or the fact that Zelda's Up-B mobility options let her punish other preemtive commitments on a read. You argue these people should shield. That would be true if that was actually always their best option.
Let me remind you that Zelda is durable, and she hits hard. She will outlast a lot of characters if she's allowed to play traditional footises simply because her reward for getting in is really high, and virtually nobody is able to edgeguard her or edgetrap her with any real consistency. She HAS to be pressured in some way to be KOed, and that's why Up-B is so good. It means your opponent has to play conservatively when trying to trap her, which makes it really easy for Zelda to get back to neutral. And if they don't play conservatively, enjoy your free kills.