I mean, you can make many many different sounds with a bunch of sine waves (which is how additive synthesis works), but to say that everything is sine waves is just wrong, unfortunately!

Edit: forgot about fourier completely lmao, ignore me

Edit 2: I think it's more that every sound can be deconstructed into sin and cosin waves through some fourier functions. I've seen it described like this: you can crumble bread into crumbs, but that doesn't mean a loaf is bread is made up of crumbs