While true in the general case, in this case the bilinear downscaling pass is being applied to a signal already previously upsampled using a known specific filter (box filter). Aliasing is therefore more limited and controlled.
"Bilinear downscaling" also doesn't make sense because scaling an image down means doing a weighted average of the multiple pixels going into a single pixel. Pixels being weighted linearly based on distance would be a triangle filter.
Aliasing is therefore more limited and controlled.
Aliasing doesn't need to happen at all with a reasonable filter width. If someone is interpolating between four pixels, that's a triangle filter with four samples.