when it spontaneously degrades, yes, it turns into tame water and healthy oxygen, but when it touches organic matter (your skin, tongue, mouth, etc) the oxygen directly reacts with the carbon atoms to make CO2, effectively "burning" away your tissues very slowly.

Usually, you don't notice that because you use store-bought 3% peroxide, but chemists regularly use the much more powerful 35% peroxide, which gives you nasty burns

peroxide burn

also, fun fact, some cells produce hydrogen peroxide as a waste product, so nature has evolved the catalase enzyme to break it down, and that's why you see bubbling when using it on a scar but not on skin, because that enzyme is only inside you and your blood

Looks unpleasant but generally not dangerous.

I think that's the point he was trying to make.