It says in the article that it could be only 6,000 out of 8,000,000, so it's not that many - I can understand it, if you got your council house when you were starting out, then gradully did better for yourself until you are on the sort of salary most of us can only dream about.
It certainly does seem to be against the spirit of council housing being provided only for less well-off people though, particularly since people starting out now are likely to need several tens of thousands of pounds as deposit before they can think about buying their own place.