ABBA made music considered to be Pop. Their records sold a lot, so were indeed popular. If they DIDN'T sell well, would they therefore not be considered Pop? Of course not. Yes, Pop is short for popular, but the use of the term Pop for a genre of music uses that term in a less-than-literal sense.
The 3 Tenors album sold more copies than most non-Classical albums. Are we to consider well-selling Classical albums Pop music?