There’s a disclaimer at the beginning:
"You might notice that, say, there isn’t any opera on our list — that’s because our purview is pop music writ large, meaning that almost all the artists on this list had significant careers as crossover stars making popular music for the masses."
Which of course makes no fucking sense at all. If they were honest, they would have written:
"We didn’t include opera singers because they have legitimate vocal skills and we don’t actually know what the fuck we’re talking about, so we left them out."