I don't know that 5,000 people were entrants at the swimming trails. If that number is accurate, I suspect that is a number taken out of context. For example, there may be numerous pre-qualifying events that weed the field down to a manageable number but I really don't know.
Regardless even if we take your 5,000 number and look at the 300,000,000 people in the United States those 5,000 are in the top .001666% of the population which is to say they swim better than 99.998333% of the population. That is an elite group of people. They worked really hard pursuing a dream and I admire them for the effort.