Um, no.
I have no quarrel with Abba insofar as them being a great pop band writing some of the catchiest tunes ever. And they're more popular than me.
But this claim is simply unsupportable.
There is no official tally of global album sales. In general, each country has some sort of authoritative source but even there are differences.
The US has an excellent authoritative source, The RIAA Gold & Platinum Database. In this database, Abba doesn't even crack the top 15.
For those who care, here are the top 15 artists via RIAA of all time:
THE BEATLES Cert. Units (In Millions): 183
GARTH BROOKS Cert. Units (In Millions): 157
ELVIS PRESLEY Cert. Units (In Millions): 139
EAGLES Cert. Units (In Millions): 120
LED ZEPPELIN Cert. Units (In Millions): 111.5
BILLY JOEL Cert. Units (In Millions): 84.5
MICHAEL JACKSON Cert. Units (In Millions): 84
ELTON JOHN Cert. Units (In Millions): 79
AC/DC Cert. Units (In Millions): 75
PINK FLOYD Cert. Units (In Millions): 75
GEORGE STRAIT Cert. Units (In Millions): 69
BARBRA STREISAND Cert. Units (In Millions): 68.5
AEROSMITH Cert. Units (In Millions): 66.5
THE ROLLING STONES Cert. Units (In Millions): 66.5
BRUCE SPRINGSTEEN Cert. Units (In Millions): 65.5
Garth Brooks 157m? The mind boggles.
This is an album list, ABBA sold mostly singles. ABBA is also big on international sales. No one pretends they are #2 in the U.S.A. I’d be interested in knowing how they sell in the Far East, a large potential audience, but distinctive tastes.