It looks like they were only looking at the NYSE and Nasdaq. Smaller companies would not qualify to trade on those exchanges, they would trade over-the-counter. There are more OTC stocks than there are that trade on NYSE/Nasdaq.
"The sample is composed of the IPOs of U.S.-based companies with an offer price of at least $5.00 and listed on the NYSE (excluding NYSE American and NYSE MKT issues after the merger in 2008) or Nasdaq (excluding Nasdaq small cap issues before October 2005 and, after Sept. 2005, Nasdaq capital market issues), excluding ADRs, unit offers, SPACs, closed-end funds, REITs, partnerships, banks and S&Ls, and stocks not listed on CRSP (CRSP includes Amex, NYSE, and NASDAQ stocks)"
"The sample is composed of the IPOs of U.S.-based companies with an offer price of at least $5.00 and listed on the NYSE (excluding NYSE American and NYSE MKT issues after the merger in 2008) or Nasdaq (excluding Nasdaq small cap issues before October 2005 and, after Sept. 2005, Nasdaq capital market issues), excluding ADRs, unit offers, SPACs, closed-end funds, REITs, partnerships, banks and S&Ls, and stocks not listed on CRSP (CRSP includes Amex, NYSE, and NASDAQ stocks)"