Okay, I know it's funny to joke about Sora being the one and only Keyblade Master in KH1 only for Keyblades to be handed out like candy throughout the rest of the series, but it still makes sense.
Keyblade wielders were already scarce before Birth by Sleep. Between BbS and KH1 the only active wielder that we know about is Mickey. That's it. He's the only one. The only ones that know about Keyblades at all(as far as who was seen in KH1) are the Final Fantasy Traverse Town group, King Triton, Ansem SoD, and Maleficent. The other Disney villains only knew because of Maleficent.
Sora is the only person in about 10 years or so to get a Keyblade. With Mickey busy in the Realm of Darkness, Sora is the only one who can stop the Heartless and protect the worlds. Between the FF gang, Triton, and Maleficent I'm sure their knowledge on the Keyblade isn't anywhere near perfect. So, especially with the FF gang, it makes sense that they call Sora the Keyblade Master as if he's the only one. Cuz in the moment he really is the only one. He's everyone's last hope.
No one knows yet how easy it can be to get a Keyblade. Barely any of them know just how many people have had a Keyblade in the past. And they sure as hell don't know how many of them are gonna be around in about two years down the line. Sora, in the moment, was the Keyblade Master because no one knew any better. So all those claims in KH1 still hold up even if it's not in the exact same way they used to.









