We use the word flat because we haven't got a specific word for 3D nor a general word to use so we use the closet one available which is 'flat' a word traditionally for 2D. Not curved/a curvature of 0, might be a less confusing way to describe the universe.

A similar issue arises when talking about 'size' in higher dimensions. An object in 1D is a line and its size is called length, in 2D you have a surface with an area, in 3D your object has a volume.

But in 4D we haven't got a unique name for its size and call it volume again. and it doesn't help that like every 3D object has a 2D surface area, a 4D object has a 3D volume. Therefore every 4D object has two measurements called volume (though its specified surface or interior volume).

And get gets worse as you go up in dimensions because every dimension you go up you get an additional type of volume to consider but the highest dimension measurement is usually considered its size.

TLDR: dimensions are confusing and we haven't got enough words in english to properly or easily talk about it.