When an opening post is flamed and all subsequent posts relate to it, then it is hard to flame only certain parts (especially when quite a few of those posts have quoted the OP).
However, it is not obligatory to flame the whole thread, AFAIK.
That said, having had a good look at it, there would have been very little left, after a LOT of weeding out had been done, so I think in this case it was the right thing to do.
Incidentally, in some cases like this, the whole thread is initially flamed so that it can get tidied up in peace, then some parts might be returned to where they came from. I wouldn’t expect that to happen here though.