My reading of the post is that he mis-typed the hhonors number (thus using someone else's). He types the (hilton assigned) PIN correctly for his sister, and it turned out that the someone else had the same PIN.

If the generated PIN's were random, then this is a one in 10,000 chance. But if they are not, then the odds become much more likely. hence, the conclusion that they may not be random, and the suggestion to choose your own.