its been a while, and i'm a little rusty
what comes before the equations and graph you posted?
what does the graph represent? (it does not look like a delta function)
the fourier transform of a delta function is 1.
the fourier transform of 1 is a delta function
best i see, is that since f(x,y) = delta(x),
then F(fx,fy) =F(f(x)) * F(f( y)) = F(delta(x)) * F(f( y)) = 1 * F(f( y))
only thing i see explaining anything, is that since f(x,y) is not y dependent, f( y) =1
apparently there is some sort of "BB" code that makes f parenthesis y close parenthesis into thumbs up emojii