Look at the October 6+ shares, before WINS started simulcasting. WCBS-AM was #10, with a 3.2 share, WINS and WABC were tied for #11 with 3.0's, and WOR was tied for #19 with a 1.5. That's not exactly "barely anyone."
In the current report, WOR's cume was 419K, WABC's was 538K, WCBS was 801K, and WINS had a whopping 1.32 Million, the 11th highest in the market. Again, that's not "barely anyone," and tons of FM stations in markets everywhere would kill to get a cume like those.
Plenty of people have AM radios, plenty of people know how to switch bands to AM. What they often don't have is good reason to do it, or a high tolerance for the noise, buzzing, fading or poor fidelity that comes along with switching over to an AM station. Where the payoff exceeds the effort, people figure out how to do it.