Some sources say that this is just the part that says that the norm of a $L_{2}$ function is the same as the norm of its Fourier transform.

Others say that this theorem actually says that the Fourier transform is bijective.

The comment at https://math.stackexchange.com/questions/446870/bijectiveness-injectiveness-and-surjectiveness-of-fourier-transformation-define/1235725#1235725 may be of interest, it says that the bijection statement is an easy consequence from the norm one, thus the confusion.

TODO does it require it to be in $L_{1}$ as well? Wikipedia https://en.wikipedia.org/w/index.php?title=Plancherel_theorem&oldid=987110841 says yes, but https://courses.maths.ox.ac.uk/node/view_material/53981 does not mention it.