The reactions you're describing are the unhealthy extremes - either extreme sadness and grief, or complete numbness and cutting off. Most parents fall somewhere in between.
I don't think anyone is saying parents can't be sad when their child leaves home, just that they shouldn't compare it to the grief of a parent whose child has died and they shouldn't make their child responsible for their own sadness or the void that's left because they've gone through a perfectly normal and expected life stage of leaving home to go to university. The parent's feelings are their own responsibility, as is gradually doing things to help adjust to the new reality.
I think on Mumsnet and other internet forums we don't get the full context of someone's life and relationships and therefore what's often missing is WHY people feel how they do.
My mum was one of those parents who immediately cleared out my room, repainted it to her own taste, etc. I think that was a reaction to the difficult teen years, she was relieved those were over and she had her life back. At the same time, I never wanted to move back home and she never wanted that either. She did support me at uni financially at a basic level, although I worked, and therefore didn't come home often except short visits - was never one of those kids that went home for the holidays. Her reaction probably would seem cold to many parents on here, but we had a terrible relationship from when I was 14-18, so in that context, it was understandable, she was protecting herself. It took about 10 years for us to heal and forge a new relationship and as adults, we're good. .