Why yeeeeesssssss, I haaaaaaaave. Remind me how that matters at all, considering we are talking about the specific selection that is being made? Here, since I was already using wikipedia as a source:

Selection bias is the bias introduced by the selection of individuals, groups, or data for analysis in such a way that the association between exposure and outcome among those selected for analysis differs from the association among those eligible`

Note that selection bias is when you are sampling. We're not talking about a sample here, we're talking about the entire population. In statistics, we call that a parameter. It's generally rare that you can accurately determine a parameter, but since the amount of people who have tried to summit everest is so small, and records have been pretty good (aye, we're certainly missing a few), that 400/13000 number is pretty damn close.