A comment in the code explaining the rationale behind this:
// the clipping is detected as >1.0 after white level normalization
* yes, technically, sensor clipping needs to be detected not accounting
* for white balance.
* but we are not after technical sensor clipping.
* pick some image that is overexposed, disable highlight clipping, apply
* negative exposure compensation. you'll see magenta highlight.
* if comment-out that ^ wb division, the module would not mark that
* area with magenta highlights as clipped, because technically
* the channels are not clipped, even though the colour is wrong.
* but we do want to see those magenta highlights marked...
[edit: I don’t find this so convincing if we consider an unbounded linear scene-referred workflow though… it only makes sense if the modules that follow are unable to cope with scene-referred values greater than 1.0…]