A couple different things could explain this. Since the Others were done with Walt, it would make sense for them to conceal Walt from Charles Widmore after Walt and Michael left the island. Then again, Walt is "special," so at this point who knows what rules apply to him in any given situation? Walt is one of the biggest wildcards on the show.
In the season 4 episode "Meet Kevin Johnson" Michael goes to his mothers house to see Walt after his attempted suicide in the car and she wont let him see Walt because of all the mystery. She says something along the lines of "I can't talk to anyone about you being alive, can't call you by your real names, you won't tell me anything about what happened to you" etc..... So yes I think they were using fake names to keep anyone from knowing who they really were and that they were on 815.