pg, knuth and schneier are practically household names, and ptacek is very googlable. If they're looking for employment, I don't think that a scanty LinkedIn profile is going to be the dealbreaker.
I think so: if your name is famous enough, hiring managers will ignore their own rules.
Also, if you went to a top tier school for your subject, most of the rules are ignored. My profile is partially completed, and I haven't updated most of it in a while. I always get a callback when I submit a resume; even if it's a position I'm grossly underqualified for.
If someone knows you, then you have a good chance of scoring an interview; otherwise it's almost impossible, at least in phd-level computer science.